She is old enough for a toddler bed. She's almost not a toddler anymore.
She is fine to sleep in the play pen.If she naps in it at all now then she's comfortable enough to sleep in it.
If you make a stop every 2-3 hours and let the kids get out and run around for a while you'll be much happier. They'll be much happier, they'll do a lot better.
I do a schedule much like this.
8am, get up, pack up, go eat. Get on the road by 9am. If you start earlier you'll need to stop for a morning snack.
Noon, stop and eat, let the kids run around.
12:30 or so pack them back up and head out again.
3pm. Stop for an afternoon snack. The little one should be hungry after napping. Don't stop until that one wakes up. Have some snacks for the other one in case the little one sleeps longer.
Then back on the road. Stop for a good solid dinner. The kids will really need time here to run crazy. They need to run off all the energy they've stored up all day. If you're spending the night at a hotel then this might be a good time to just stay put. You can put the swim suits on and go play in the pool and get them good and worn out. They can also use the cool water to reduce any inflammation from sitting in that same position all day. They're on vacation too and getting to play is a great time for bonding and fun they'll always remember.
If you continue on the road another few hours after dinner then you need to stop around 8 and have a good snack. Get teeth brushed and jammies on at the location you stop at. They'll still need some play time though.
Then you can get back in the vehicle and drive all night if you're up to it. They'll be asleep before too long and okay to sit there. I always put the car seats in a slightly reclining position so their heads would not bob around.
If you leave your home in the evening after dinner the kids can already be in jammies and have some simple snack foods to eat. They'll go to sleep and you can drive as long as you need here too.
There are lots of options but movies are the best. They last over an hour and they completely entertain them. You can drive that long and have pretty much peace and quiet until the credits start. This is a good cue to stopping too. After each movie is over you have a natural time for a break.