Take the kids to Disneyland without the airport ordeal. The Eurostar gets you surprisingly close.
There are two ways to reach Disneyland Paris from London by rail:
Eurostar occasionally runs a direct service from London St Pancras to Marne-la-Vallée–Chessy, the station at the gates of Disneyland Paris. When available, this is the best option — no changes, no navigation through Paris, and you arrive literally at the park entrance.
This service typically runs during peak holiday periods (half-terms, Easter, summer, Christmas). Check Eurostar's website for seasonal availability.
Take any Eurostar to Paris Gare du Nord, then connect to Disneyland via the RER A regional train:
Total from Gare du Nord: about 45–55 minutes including the change. A single RER ticket costs around €5 for adults.