Our child is almost 3 years old and it is a bit tight at the bottom, but it doesn't bother him and at that age they don't sit in the car for hours. So it's absolutely no problem for everyday use. As long as my baby is still small enough (now 2 months), I fold up the foot part of the tub a little. That way, it doesn't bother my son as much and my baby still lies flat, as the tub is easily big enough.
The sibling can stand up in the lower seat at times, so it still rides wonderfully.

Our child is also 3 years old and has almost no room in the back with his head up. But it's still fine for sleeping. I think getting in and out is more difficult with the carrycot. But for this price, a compromise is possible.
Have a look at the manufacturer's homepage, maybe you'll find more information there.