So, comparing connections from either Birmingham or Manchester, which has the most straight forward rail connection to Hull? Does either one have a direct (not transferring from one train to another) connection?
Neither do - both Birmingham International and Manchester Airport require train changes to get to Hull.
I am a little nervous about them making a connection from Schipol back to the UK, although it would be much closer to their ultimate destination. If it were me, yeah, but not this pair. They'd be prone to get lost in the shuffle :-)
If it was me, I'd strongly recommend this route - it will be much easier to change in an airport than to deal with the UK train system, particularly as the UK train system can melt down at times. The other thing is that if they have problems in Schipol, they'll get plenty of help (including if they miss their connection) - whereas a delay into the UK means they could miss their train and so on. If they're prone to getting confused and mixed up, then Schipol will be far friendlier than the UK :)
I have heard that Heathrow is quite a confuzzling kind of airport, and am a pretty sure they wouldn't make it out of there with their sanity (based on what I've heard.....)
It's not that bad, but the real problem is transiting London.