The connection from Paris to DLP is by the RER A which is just a 5-zone ticket
So when you land, you can take RER B into Paris, then transfer to Gare de Lyon by RER E. Given that there are two of you with a 4 yo. Since the cost will be just under 28 EUR and a fair amount of walking at the stations, consider taking a fixed price taxi from the airport taxi line to St Lazare station for 50 EUR and the taxi can drop you right in front of the station.
There are direct trains from St Lazare the Bayeux at 1:10 pm and 4:04 pm, and a train with a connection in between. The 1:10 pm is 34 EUR per adult and 17 EUR for the 4 yo. You could buy them now, but if you ran seriously rate, you would have to get them changed for a fee at the airport train station, but this has to be done before the train departs. So I think best to just wait and buy tickets at the station. 4 yo ticket is half of what ever you pay.
Return to Paris you can buy nonrefundable tickets now back to Paris St Lazare for 59.50 for an 8:33 am departure. www.trainline.eu is a good site to buy the tickets.
At St Lazare, there is a circuitous underground passageway to Auber RER station where you get the RER A to DLP. Tickets are 7.60 per adult and 3.80 for the child. Same price for the return to Paris. Trains a very frequent, at least every 15 minutes, probably more frequent. Since the line splits half way to Disneyland, make sure you get a train going to Marne-La Vallee-Chessy. There are monitors on the platform with the destination so you can see that it is the right train.