Disclosure: I received complimentary products to facilitate this post. All opinions are my own.
There are an awful lot of lap desks out there, each with nifty features. One of the options that really rises to the top is ZoomKIT. It's comfortable (it says it fits up to kids who wear a size 8; my older daughter wears an 8 and still has some room for comfort if she grows a bit). It is convenient - it's a great way to pack and store what's needed for long road trips. Plus, it's customizable. There are several different inserts that are easy to swap in and out for different needs - playing, snacking, even an art kit that fits neatly right inside.
Honestly, my only complaint is that it came after all of our road trips this summer. However, it's going to be great for homeschooling, since my girls currently share a desk - now they can both be writing at the same time, without bumping elbows on the small desk. It's also easy to bring to other places, like sports practice or other times where kids have to wait around. It looks sturdy and well-made, and the pieces fit together well so there's less chance of spills on a bumpy road. And it's designed to be as safe as possible, with rounded edges and flexible material, and collaboration with crash test safety experts.
Currently your best bet is to order online, although hopefully you'll be able to find a local store in your area soon!