How long does it take to do the highlights in each park? We're considering adding two days to our vacation and hitting US and IOA. I have two kiddos, DS8, and DD5. Are they old enough to really enjoy the attractions? Or am I better off waiting a year or two? I don't mind spending the extra money (it would be quite a bit as we would stay onsite) if they would really have a blast, but don't want to go if we are better off waiting or wouldn't be able to cover much in two days. Thanks for any info and opinions!