I like Cortex. You can download Cortex from DriveThruRPG for five bucks.

Willpower could be renamed Bravery, and Intelligence become Imagination. Alertness, Agility, Strength, and Vitality can keep their names.

Weapons can be based on the toy. Teddy bears could be unarmed, or the kid could give them a small wooden sword and small helmet, which the bear can turn into an actual sword (albeit small enough for the bear to use it), which could be an English Longsword, a scimitar, a katana, or a spatha, and the helmet could turn into a suit of plate, or an iron breastplate and tower shield, or a bronze breastplate and round shield. Army men can have assault rifles, grenades, even a sniper with a sniper rifle, and are well suited to larger parties. Action figures can be like army men, except they're generally better equipped, and can have a wider variety of weapons and fighting styles. An action figure could know kung fu and dual-wield assault rifles, or carry a bolt-action rifle and a rocket launcher.

The scale rules should be tossed, though. It's just not good for the game if the monster deals ten times his normal damage to the toy, and the toy has to deal at least ten damage just to do anything, and then only one actual point for every ten rolled.

I don't have Mouseguard, but I can't shake the feeling that it would be a good fit.