There are very few local travel agents left these days and I doubt that any of them could dedicate as much time to a single enquiry as people on here can!
Rail travel from Leeds isn't unrealistic (and it would be far, far less hassle than driving). You can book through tickets all the way with Eurostar but the current booking horizon only goes as far as October.
So I've used prices for outbound travel on Monday 3rd October, returning a week later, to get a sample price for you. Based upon 5 adults, one 7yo child and one 2yo, the total outbound fare (all the way from Leeds to Disneyland Paris) is £50.00 per adult and £32.50 for the 7yo. (The 2yo goes free if sitting on an adult's lap. Add on a further £32.50 if a seat is required). So that's just £282.50 to get all 7 people to the resort.
That's based upon leaving Leeds at 1145, arriving at Kings Cross at 1358. Then taking the Underground (which isn't included in the Eurostar price) to St Pancras, for the 1504 service to Liile Europe, arriving at 1726. Finally, taking the 1752 service from there, arriving at the resort at 1857. (Other services are available but are slightly dearer. For example, travelling a couple of hours earlier puts the adult fare up to £60.00 and the child fare to £39.50)
Coming back you can also do it for £282.50 (for all 7 people) if you don't mind getting into Leeds at 0234 (after leaving the resort at 1852) but it's a much quicker journey if you leave at 1552 (arriving in Leeds at 2117). For that you'd pay £60.00 per adult and £39.50 for the child.
There's also a direct train from the resort to St Pancras at 1802 (with the connection getting you into Leeds at 2257) but that's dearer, at £81.50 per adult and £54.50 for the child).
So, if we assume £282.50 outbound (on the 1145 service ex Leeds) and £339.50 back (travelling at 1552 from the resort), that's a total of £622 to get all seven people there and back. (You'd just need to add on your London Underground fares and the costs of getting to Leeds Station, as well as the cost of parking there if you needed to).
http://www.eurostar.com/uk-en
If you preferred to fly, the only service to Paris from Leeds-Bradford airport is with Jet2 (with flights operating on Sundays,Mondays, Thursdays and Fridays). Based upon sample dates of out on Monday 6 March and back a week later, I'm seeing the following:
Out at 1100, arriving Paris CDG at 1330.
5 adults at £34.40 and 2 children at £24.00. Total £220.00.
Back at 1415, arriving 1445.
5 adults at £41.60 and 2 children also at £41.60. Total £291.20.
(The reason the child fares are lower outbound is because the UK doesn't charge Air Passenger Duty for children but there are no similar savings when returning).
So that's a total of £511.20 for your air fares but you'd need to add on the costs of getting to the airport and possibly the cost of parking there for a week). You'd also need to add on the cost of getting to the resort. Travelling by train a typical cost is 6 x £10.50 each way (with the older child paying the same as an adult and the younger one travelling free), so that's £126.00 in total. Adding that onto your air fares gives a total of £637.20 to get everyone to and from the resort.
SNCF website:
https://uk.voyages-sncf.com/en/#/
(Note that it's not yet showing times and prices for next spring. I used dates in October to find some sample prices)
So there's very little in it (in terms of price) whether travelling by rail or plane. The shorter journey time by air might push you towards going that way but the costs of airport parking (or just the experience of Eurostar) might make you decide to go by rail. (It's also worth remembering that the flights don't operate on every day of the week; you'd need to consider whether there are flights that tie in with your other arrangements).
Then you need to look at the 'early booking' offers that are currently available from Disneyland Paris:
http://holidays.disneylandparis.co.uk/offers