My 7-year old wanted a chocolate chip filling in his bday cake but I made it last night and it's pretty sweet -- I'm worried it'll be too sweet with a chocolate frosting, and the cake must have chocolate frosting. I had put it with a marble cake, but I'm willing to change the cake and the filling. Any suggestions would be really helpful!

How about making a chocolate chip cookie the size of your cake and putting that in the middle.

Not sure what recipie you used, but I made this one for my son's cake and we all loved it.

http://cakecentral.com/recipes/7600/chocolate-chip-cookie-dough-icingfilling

If that's what you already used, then I second amateurnw41's idea that a big cookie in the middle sounds yummy!

Good Luck!