I second the free stickers. You can get them everywhere and the CMs have no problem giving you a bunch to pass out to the kids at home. My grandson loves to bring them back to the kids in his class and they love stickers. There are two different Mickey designs that are easy to find and there are other designs that I have only seen a few times. The booths at the parks for
DVC have stickers but they are advertisement type for Aulauni in Hawaii. Sometimes the monorail drivers and the CMs standing at the monorail will have transportation cards and monorail "wings" stickers.
You can also go to MK each day and get a packet of Sorcerer cards. they are free at the old FireHouse. There are 60 different cards but you get a pack of 5 each day for each admission ticket you have so if you go with others you can get enough for all the kids in a few days. One or two cards would be cute. They are character photo cards with powers etc... written on them, kind of like the "Magic Cards" the teens collect.
Pop and All Star food courts sell Mickey straws but they would be expensive for all the kids. I remember them running about $.50 each.
I would check other places for Disney type pencils. They are more expensive at the parks.
Now here is a silly souvenier, not sure if they would give you a bunch, but my grandson loves the kids meal menus from all the sit down restaraunts. They have games and word search, puzzles, jokes etc all printed on them. It wouldn't hurt to ask.
That is all I can think of right now, I will ask my grandson in the morning what other little free things he has gotten at the parks and let you know.