My girls want to add an animal to the 10 gal terrarium we made. I have no ideas what might be suitable. Any suggestions besides spiders?

No see the frogs could be the food for the neps (survival of the fiteset)
By spiders, do you mean taranchulas? I have one and wouldn't recommend them. I gave one to my son's friend who unfortunately got some of the spider's hairs (microscopic ones) in his eye which are not removable. He's on steroid drops now.
how about mosquito larvae some mosquitoes lay there larvae in certain species of nepenthes pitchers.