It's really weird looking, but my dad bought me one of those strap on head lights like miners wear and it was life changing for me. I also had one of the Pottery Barn lap desks because I found that to be much more comfortable than just using a notebook to write on. Those might be something to look into for Christmas presents for your girls as they're kind of pricy. I will say it was worth the investment for me because I used it all through HS and college.
For worksheets and math, just do the best you can. It takes a little longer, but I'm a perfectionist and would sometimes do my work on one paper and then rewrite it out of the car. For anything written over a paragraph long, just type it. My parents got me a super crappy laptop on Black Friday for like $150 and it was perfect for writing papers in the car. I get carsick so if I was going to be reading a lot sitting in the front seat helped, but if writing sitting in the back was better. Keep mints/gum/suckers on hand. If you eat dinner in the car a lot, get shower caddies from the dollar store. They'll hold all the food without spilling and catch crumbs. If you haven't already, throw an extra notebook, pencil, and calculator in the car. It's super frustrating to not be able to do anything because of forgetting supplies.
It's hard at first, but they'll get used to it. Also use any down time at the gym wisely. I always used to read while stretching at the start of practice. Tell teachers that on days following practice days the homework might look a little messy, but make up for it with perfect homework on the other days so they'll cut you some slack. Good luck to your girls! And way to be an awesome mom and ask these questions!!