From London is relatively easy, Eurostar from St Pancras station is probably the best bet. It is fast, reliable and direct. Normally leaving London at 09.53 daily arriving at the DisneylandParis resort station (Marne La Vallee) at 13:31 (French time), probably always best to double check times and dates of trains in advance as these can sometimes change.
On the return train journey the train departs at 19:37 most evenings and arrives back in London St Pancras at 21:12. Really convenient, fast and efficient especially when you use the 'Disney Express' service at Eurostar this will help cut your check in time. There are normally Disney Cast members on board the train so the kids can be entertained throughout the journey. The Disney Express service is usually available for specific package holidays, so also important to check how you can qualify for this.
You can drive to DisneylandParisas well, however, that will probably take a little longer. Once you have arranged your crossing from Dover in the south of England to Calais in France by ferry, or Eurotunnel. The A4 motorway in France is the only major route you need to be on, and is sign posted all the way to the Disney resort.
Parking is normally free if you are staying at one of the DisneylandParis Hotels nearby; however, if you are not then parking charges will apply and may vary throughout the day.
Taking the coach from London to Paris can also be fun and cheap! Eurolines are offering tickets from £19 so it is a fairly economical way of getting around Europe. Alternatively you could take your own car through Eurotunnel, but it means you have to do all the driving, although it allows you the convenience of moving around the country. Expedia provide a range of special offers for travel not just to and from France but to Florida in America or Hong Kong in China - there is plenty of variety and Expediaare fairly comprehensive in the options open to travellers.