Wow that is a very lovely gesture! I'm sure everyone here will be able to help you out - the first thing you need to consider is if you would like to book them a package staying offsite or on site. In my experience to book offsite you get better deals on the hotel booking through expedia or bookings.com but you then need to buy tickets too. If you would like to book onsite the prices quoted on the DLP website include your tickets and in my opinion booking direct with DLP (when there is an offer on) is always the cheapest option (unless they have APs but judging by your post I'm going to assume not).
Now if you could tell when you would like to book the holiday for someone will be able to give you a rough idea of when an offer for that time may come out.
Also - I've never travelled by eurostar but I know other DISers on here book that seperatly to get the best price - the earlier you book the better the price so if you have the holiday dates in mind this might be the first thing you want to book. Bear in mind that DLP is always more expensive during the school holidays - both English and French.
I agree with Jonjo, book the hotel and tickets direct with Disney (or pop into First Choice as they will book direct with Disney also and it will cost you the same) and book the train tickets direct with Eurostar.
You are just up the road from me, i was shopping in Southend today.