If you board with family boarding, you’ll have no trouble getting a window seat. I’ve never heard a request for someone to move for a family needing a window seat for a car seat. The only situation I can see a problem is if you have a connecting flight that is delayed & you’re one of the last boarding.
If you’re concerned about Mears not getting you to the airport early enough, just tell them a flight time about a half hour before your actual flight, so you get picked up earlier. I do it all the time with a private car service. We’re 2 adults with TSA Precheck, but we prefer to be at the airport earlier & not stress about traffic or delayed driver. So I always just give a flight time about 20 minutes before our actual flight. That way you‘ll be there in plenty of time to make family boarding.