My daughter is only 8 weeks old, so she is still in a baby seat, but we went for a 650km round trip yesterday and she slept for 5 1/2 hours out of the six hours we were driving. She hasn't had a problem since the first day we drove her home.
For a 2 1/2 year old - I must admit that I don't know, as their seating position would be different. I don't think that it would be too harsh for them, but maybe the lack of space might be a problem.