I have had it happen twice... Which I will get to in a moment.
In the case where you mentioned your paladin killing a defenceless boy... That really isn't 'the fall' IMO. Now it is certainly something that would make him lose his divine abilities... but it is also something that he could atone for and continue on his paladiny way.
For the cases I have DMed and a paladin fell the first was expected. We were playing a neutral game (but wanting to play an evil one) The paladin character wanted to become a blackguard, so when we reached an appropriate point in the game... he suffered a trauma (his wife and son killed by another paladin being tested by their god). He went on a rampage killing the offending paladin forgoing his religion and becoming a blackguard. And we became an evil campaign.
The second time it happened it was a campaign that had been running for some time. The paladin wanted to change classes... so started doing things against his code of honour. In the end he fell because his alignment changed and didn't bother to atone. Then on his next level took a level in sorcerer.
Now in both of the above cases (while they wanted to be paladins) the players made some bad choices (much like what you mentioned with your game) and it just gave a new dimension to the game, because now they had to atone for their choice. Each time it was a reason for the next adventure, and any pall the fall may have caused was forgotten with the act of atonement.