Oooh, Small, it's not good - ouchy prices :(
I think 3 nights/4 days is the best option - it will be pretty busy and you won't get everything done if you only go for 2 nights/3 days. Here are the prices for the hotel I would choose (the one I told you about when I sent the info before - the Cheyenne). First price is for 2 nghts, 2nd price is for 3 nights:
Leaving on Monday 18th or Tuesday 19th it's £714 or £992.
Leaving on Wed 20th it's £642 or £892 .
Leaving on Thursday it's £571 or £956.
Staying at the cheapest hotel (Santa Fe) will save you between £10 and £40 in total, depending on the date, but I don't think it's worth it as the Cheyenne is MUCH nicer.
The above prices are just for the hotel & park tickets - transport is extra. I priced up the Eurostar for the Wednesday departure 3 night option and that brings the holiday total to £1225 (I think the Eurostar adds on about the same amount, regardless of date, so you're looking at adding on about £335 for the train - may be less if Smallboy Minor sits on your lap but I wouldn't recommend that).
You can price up various other date/travel combinations easily here
Oh and if you sign up with topcashback.co.uk you can get a sizeable chunk back and it doesn't cost you anything to do - I think it you would get about £50-70. Oh and if you're going to sign up with them then please let me know, as I can earn extra cashback by sending you my referral link :)
Hope all that helps!