It's THEIR cake, not Martha Stewart's. Use white fondant to cover your cake and give your child food markers to decorate their cake. Imagine doing this every year and seeing (in pictures) how your child develops.
...it´s been a long time since my kids were young enough to need to be entertained at a restaurant but these ideas do bring back memories! ;) Perhaps they´ll be helpful for you as well! 10 Restaurant waiting Games to Play with Kids