If they can go w/c 11th June then I would strongly advise them to go for then, it will be quieter as us lot will have left by then

.
Where to stay depends on their budget - if they can afford it the DLH, if not, another onsite hotel to suit their budget. I would recommend onsite for at least a first visit, as someone else said, it adds to the magic, plus it can be a bit daunting sorting out tickets/APs for the first visit
When to book - now with the 40% off offer if going in June (think the current offer includes 11th June, but doesn't include half term!). Not sure there will be anything better now that comes up for June now
If they can go later on, they could hang on to book hotel....
How long to stay, again depends on number of party/children/ages etc. I would say 4 nights for the first trip as an average.
HTH, not sure of their budget or their party makeup so can't be more specific

.