Children & low fidelity testing: Possible?

I've been working for a children's educational start-up for the last few months and struggling to fit user testing into our process.

Typically, when we want to create a new feature we'll do some interviews and observation, build a few concepts... and then rush straight to design & build. There's a few factors why the testing gets skipped, but among the largest is a lack of confidence children will be able to give useful feedback without very high fidelity designs (which can take as long as build does for us).

The problem is not so much "No user testing OMG!" as it is many conversations about UI design that are fairly subjective without real user input (Will they like this layout? colors? extc). Anyone have any advice?