An idea I've heard from other travellers: buy small items, play with them at the hotel, give them away before returning home. Your kids will have travel toys that are different than theirs at home, and you will have extra space in your suitcases for souvenirs when you return.
At the dollar store you can find boxes of 1000 stickers, barbie type dolls, packages of plastic army men or dinosaurs. If your kids really like something, it's easy to replace once you return home. You can find a nearby orphanage to donate your slightly used toys, or give them to a hotel worker who has been helpful to you. They can give your used toys to their children or sell them for extra cash.