Having the Children play the adventures.
Perhaps they are orphans in the local orphan house and they decide to play "The knight saviors of Darkon". So one team are the "undead" and the other team is "the knights". Remember in D&D everything is just a comparison with the normal. If you say that in this campaign, strength 10 = the strength of the average 10 years old boy then you have your new set of rules.
Be ready to improvise a lot however. This needs much work.
You beef up their skills, hp etc as if they are say 2nd level characters, but make everything in the "real" world difficult. So a "rogue" kid with a hide/move silently modifier +8 doesn't actually have a +8 modifier.
Once in a while, drop in a real problem. A goblin! He sneaked in the kitchen to eat the cakes!
Examples of how I would upgrade the normal creatures of the world:
You'll notice that certain things seem much more reliable than usual.
Goblin:
Medium humanoid. (That is 3' 6" tall.
)HD 6d8 +6 (33 hp)
AC 18 (+2 dex, +4 leather armor, +2 natural armor)
Attacks: Slam attack +6 (1d4+2) or club +6 (1d8+2) [Note the club is actually a broken chair's leg!)
Abilities: Str 14 dex 14 con 12 int 11 wis 12 cha 8
Skills: move silently +10, hide +7, spot +6, listen +6 (I haven't calculated these)
Feats: choose 3 feats.
"The Ogre" (Actually the 13 years old, bad boy that bullies the PCs)
Large humanoid. (5' 2" tall)
HD 4d8 +11 (29)
AC 13 (-1 size, +1 dex, +2 natural armor, +1 thick clothes)
Attacks: Slam attack +3 (1d4 +2) or by weapon +3
Abilities: Str 15 dex 12 con 14 int 10 wis 10 cha 9
Skills: move silently +5 hide +4 spot +5 listen +5
Feats: power attack, toughness, +1 feat
"The Guardian" (The 20 years old maid that runs after the kids, skilled at driving away goblins with her broomstick)
Large humanoid. (5' 6" tall)
HD 7d8 +17 (50 hp)
AC 14 (-1 sz, +1 dex, +3 natural armor, +1 thick clothes)
Attacks: Slam attack +7 (1d6 +3) or club +7 (1d10 +3) or the broom of tears +8 [a normal broom, held 2 handed] (2d8+4)
abilities: Str 17 dex 12 con 14 int 12 wis 14 cha 12
Skills: Spot +10 listen +10 profession (orphan house keeper) +5, profession (cooker) +6, intimidate +6
Feats: toughness, power attack, Weapon focus (broom)
Special abilities:
Protector: The Guardian holding the broomstick is a fearsome opponent when protecting the orphans from real harm. She gets a +2 to damage when defending the orphans (Woe to all goblins that think they can get away with stealing children pies!)
Broom Cleave: Being forced to face more than one opponent at a time (perhaps two dogs, two goblins etc) the Guardian gets 1 cleave attack 1 per encounter as if she had the cleave feat.
Scream of Rage: 1/day the Guardian may evoke a terrible scream. Commands screamed or immediately following the scream receive a +5 circumstance modifier to the intimidate check.
(Examples: "GET OUT OF THE KITCHEEEEEN! ! !" // "To your beds NOOOW! ! !" etc.))
A silly movie with silly charactyers but still a RavenLoft-escque feel