Baby-friendly cake frosting? Looking for creative ideas

Hi there, I'm cross-posting this from /r/baking but I'm in desperate need of some help and thinking outside of the box.

I'm making a cake for my nephew's first birthday, and I'm thinking of alternative ways for frosting without using sugar or dairy products (mom is still slowly introducing new foods to him). Unfortunately he has not tried coconut yet, so I'm not able to use coconut milk as an ingredient as of this time.

Call me crazy, but I'm thinking of making baby food purée (and arrowroot starch for thickening) as frosting, but naked-cake-style since the purée won't stay in place. Is this in any realm of possibility? I tried to search online to see if it's done before but to no avail. If anyone has any suggestions, I'd greatly appreciate it. Thanks!

P.S. This is the recipe I'm using for the cake itself: http://www.fitmamarealfood.com/babys-first-smash-cake-healthy-no-sugar-banana-cake/[1]