eldest can do it all herself (open/close car door, in and out of seat, fasten and unfasten seatbelt)
dd2 is 3 and she can climb in and out of her seat, but can't buckle or unbuckle her clip (like above, I don't think she has ever actually tried!)
2yr old can climb in herself but thats about it! She probably could get out but I always leave her till last so I just lift her out myself