I don’t know much about appropriate fillings for a fondant-covered cake, so I’ll leave that to the others.
However, I do make lots and lots of cakes for kids. Even the cakes I make for adults generally get sampled by my daughter and her friends, so I hear a lot of opinions. In my experience, most kids prefer the butter cakes. I know Hector has made a fabulous Elmo cake, I think from Biscuit de Savoie, but my daughter prefers the butter recipes. (Then again, one of my friends asked her what her favorite food was, her reply? Butter!)
For a two-year old, I’d keep it very light and simple with a yellow or sour cream butter cake and some fruit (do you know what fruit the child likes?). Hector made his Elmo with strawberries, it looked like just what you’d hope to serve kids. Is fondant non-negotiable?
I filled my daughter’s yellow birthday cake with Rose’s strawberry puree, slightly thickened with a little gelatin, they all loved it and seconds were had by many.
Good Luck, report back!