I am hoping that some of you could help me come up with some ideas.

I am DM for a tabletop D&D game for some friends and their kids (ages 10 and 11). There are a lot of things going on in the game, but the next adventure seems to be falling into place as follows:

(First a little context - the game is using the map of the Sword Coast in the Forgotten Realms, but beyond the locations of major landmarks, is only loosely based on it. The woods in question is technically Neverwinter Wood.)

Ok, so the plan is that the group is trying to discover what has made the woods "go bad." (6th level) The basic idea is that the woods is home to a group of Goliath Apes (think King Kong) - these creatures are no problem in general as they are not inherently violent, but maintaining a breeding population requires some trickery. Specifically, the giant apes can create gates to other planes (Normal planes like this one, nothing fancy) where they can mix with other populations of giant apes which share a similar ability. Anyway, an evil race of "monkey men" from one of these other planes has discovered this ability and managed to hold one of the gates open after the amorous apes were done with it. They are using this gate to invade and are the source of the fouling of the woods.

Simple, right?
Well, the goal of the party will be to destroy the gate thereby preventing further incursions. What I can't figure out is how to destroy the gate. I'd like to do something more clever than "destroy the stone archway." Any suggestions? Plus, the possibility exists that if a gate could be opened in this way once, does closing this one gate really solve the problem?

Any suggestions?
Any suggestions for the woods in general or how to create a monkey city?

Perhaps you could have the group (after finding out about the gates) could go on a quest to find the specific ape which opened the gate, and/or find a specific item to be able to disrupt the gate.
If it is a specific item perhaps make it a religious artifact, which would then prevent future gates from being used in this fashion.

Anyone else have any ideas?

I'm thinking I might force then to go through the gate to destroy it, but then how do I get them back?

Come on, I know this site is loaded with extremely creative people!

Neverwinter Woods is home to a Skymirror. An ancient and powerful scrying device that the Circle of the Woods (a druid conclave) uses to contact the elders of other Circles, or fey spirits of other realms for guidance and information.

The Skymirror is literally a lake in the center of the woods, surrounded by four great oaks which bear clues symbollic of druidic spells which need to be cast upon the trees to unlock the wards denying access to the skymirror.
All the spells are second level spells so well within reach of 6th level players.

Often one tree has a carving representing a spell. Such as an owls face, (Owl's wisdom) a charging bull (bull's strength), a jaguar leaping (cat's grace) , maybe a half human half bark face (barkskin) etc

spells easily chosen by a druid/ cleric, or in some cases, arcane spell casters. of course you can set the clues yourself.

Once unlocked, the skymirror may be able to 'guide them through a projection above the stillness of the lake, (imagine a reflection in the waters) of where the Gate is. As for closing it.

Have 1-2 monkey men casters as maintaining it. Once defeated the gate will close, within however many rounds (as it shrinks and dissipates) allowing potential escape time if they have to close it from the other side.

This owuld lead to a likely 'boss scenario' where the adventurers have to fight their way through guardians to these gatekeepers etc to stop it. (imagine storming a stargate if you followed the show)

Or alternatively let the skymirror show them of a Lodestone, or some other named relic that has the power to seal a gate indefinately. It could then set them on the path for an entire adventure to locate the object AND should they deign to use it. (maybe as an option other than going in to stop the gatekeepers) it adds for future dynamics as perhaps now the Great Apes are cut off from their 'breeding grounds' and it could have in game consequences for the ecology of the species or Neverwinter Woods as a whole.

just a few quick thoughts on a break from work.

Well, when the apes that created the gate open gates in general, HOW do they do it? What is the rough magical mechanics of it? My idea: they bend the local ley lines into a nexus point to power the gate. Maybe they normally 'snap back' to their regular positions eventually, but this time the EVil Ape Men did something to keep the ley lines where they now are, to keep the gate open.

I would have the Evil Ape Men use certain items to deflect/change the ley line flow. The PCs would have top trace the ley lines back and find the items and SAFELY remove them. (How easy or difficult this is would be up to you; a simple application of spells to a quest to find the right tool to remove the item.)

Have this happen to, say...4 ley lines (one for each magical element of Earth, Air, Fire and Water), or however many you like; each ley line would have at least 1 item placed to move them.

I would suggest enchanted spikes of some rare, valuable metal that are at LEAST 3' long, and a hammer of another type of rare material to strike them with, that would send vibrations to affect the ley line; when the spikes are removed, the vibration interference is stopped, and the ley line returns to normal in a few days.

After the quest ends, they can keep the spikes and hammer (which can be de-magiced when not in use with a ley line) as booty.

I hope I was clear...this is all just off the cuff!

I was already assuming the Druids would be the source of information, but hadn't heard of the mirror pool. Sounds cool. The plan might be coming together. Ley lines are also interesting. Thanks.

I'm open to more creativity if anyone wants to add to this!