I would say, yes the metro and train are very easy if you are just adults with little or no luggage, otherwise it can be quite difficult depending on the stations you have to use. Many stations have lots and lots of steps and no lifts and trust me as someone who has done this before with luggage and a child in tow that it is NOT easy.
I know it is much cheaper to use the metro and the train but with a 2 year old and luggage I would advise a shuttle (not a taxi) which is what I did last year. I used Tio Shuttle from Paris to DLP and found them excellent but other posters on disboards have used Ezy Shuttle and RS Transports so you can shop around and see who gives you the best price. Shuttles have a set price so you can budget in advance, unlike a taxi who will charge you on a meter. As far as I remember it should be in the region of about 60 euro to get from Paris to DLP. Yes it is much more expensive but if you can afford it it is a much more relaxing option.
Really consider if both of you can carry a 2 year old and a push-chair and lugguage up and down several flights of stairs. Sometimes in the same metro station you can find yourself going up a flight of stairs, walking down a corridor a bit and then going back down the same amount of stairs in order to get to your train. You then may have to do the same when you change over from the Metro to the RER.
I hope you have a brilliant time! Paris is a wonderful city and then you still have DLP to look forward to afterwards!