I prefer booking on-line but I know a few people who have tried booking this way and still ended up having to ring up to make the booking (tech difficulties etc). I’d say get your quote on-line (usually there is a discount for cutting out the middle man) and then ring up and see if they give you anything different. Keep a note of all the prices, accoms, packages etc they price you for and then go over them and see what best fits your family/budget.
It sounds like a lot of hard work and effort, and in some ways it is, but it’ll be worth it in the end!
I’d say the only problem you might face in booking a different resort and then wanting to change later down the line is the additional fees you might be faced with, from the variation in price of the resorts to possible admin fees etc. I’d always recommend people have a back up/second choice resort so you won’t be totally disappointed if you don’t get your first choice!