My advice re going there at Easter - go early and arrive there asap after the park opens.
Pace yourselves, wear soft and comfy shoes and try not to wear yourselves out!.
Take half the clothes and twice the money!.
Do not rely on UA to fully provide on the flight; take their favourite foodstuffs with you for them to eat. Also if you have one use a portable DVD player (inflight entertainment can be poor for children).
If you have a photocard based driving licence take this with you as some stores do like to see picture id when making purchases.
Picture ID can be asked for also if you wish to buy an alcoholic drink.
Do also inform the credit card company of your travel plans as your credit card may have a "stop" marker put on it otherwise.
Be very wary if using a mobile phone to call the UK; cost can be extravegent. Buy a phonecard instead (you can buy these from supermarkets and you will get a lot of talktime for around $10).
Universal now offer a front of line pass at a price but this does not actually take you to the front; it may just put you on a shorter line instead. Do the "popular" rides first; for example at Universal Islands of Adventure you may want to ride the Incredible Hulk coaster or the Spiderman rides first.
Do look at their websites and pay close heed to their height requirements for rides. They will not let the child ride if he/she is not tall enough.
If the anti sickness medication is in tablet form this will pose no problems. If it is in liquid form then the 100ml rule applies; I would get a letter from the GP if this is the case and hopefully this will be let through security without problems.
The ESTA is now a mandatory requirement for entry into the USA so if you have not done so already do complete this online prior to travel. The website to use is this official US govt one (only use this one because ESTA is free). Its esta.cbp.dhs.gov.
This will need to be completed for all members of your party.
If you turn up at the UK airport without having done the ESTA beforehand (and they will know) they may not let you fly.
any other questions just ask.