Why would you need anything different for children? I thought you were going to say you always use liqueur in your fillings and need something non-alcoholic. 
But most kids love BC-as-filling, any flavor. Fruit fillings, ganache or anything else you would normally use is fine too although for young kids you might want to go more sweet, rather than more tart (fruit) or dark/bitter (chocolate).
For plain yellow cake I'd probably go with vanilla, chocolate or a berry flavor. Or for something more unusual, maybe cinnamon/nutmeg/ginger type spice(s) in the BC.
My two most requested for kids are strawberry and banana mousse with fresh banana slices.
Yellow cake with vanilla buttercream is great for kids -- everybody likes it. I think kids all love peanut butter/chocolate filling, too, but unless you know the kids, the peanut allergy thing can be a problem. My kids' school won't allow anything with nuts in the building, so I guess it's a very big deal.
Cookies n cream is my most popular filling flavor for kids. I just smash up some Oreos but make sure to leave plenty of big chunks and mix it into my regular buttercream. Easy & delish!
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