Need help choosing transportation - CDG->Bayeux->DLP->Paris

My first comment is that Charles de Gaulle airport and Disneyland Paris are practically next door to each other, on the same side of Paris. So it makes sense to do Disneyland first or last. The order you suggest (CDG->Bayeux->DLP->Paris->CDG) is zig-zagging back and forth.
Point 2, the Bayeux area (Normandy) is good for touring by car, but you already have a tour booked (is that a bus tour?) so a car is superfluous.
To access Bayeux by train you have to go via central Paris, and it is not good to pre-book a train after a long flight, as you cannot predict when you will get out of the airport. I would have suggested CDG->Paris->Bayeux->Disneyland->CDG or CDG->Disneyland->Bayeux->Paris->CDG

Unfortunately, since you already have your tour in Bayeux, and Disneyland booked, you cannot change the order. You have painted yourselves into a corner.
Conclusion: after you land, head straight for Bayeux by train, which involves going via central Paris and using the métro.
Pre-book your tickets fro the return leg Bayeux-> Disneyland on Monday, which will also require crossing Paris.

Thanks. The reason we booked it that way is because Disneyland is busier on the weekends. Since this will probably be the only time we go there, we didn't want to deal with the weekend crowds. I'm on a few DLP Facebook pages, and chose our dates based on when the crowd is expected to be lowest. So even though it's not the best route as far as ground transportation goes, it's worth it for us.

The Bayeux tour is a bus tour. My husband is a history buff who is also in the military, so the only thing he asked for during the whole trip is a tour of the Normandy region. He will be full of questions for our guide :)

While you are in Bayeux, try and find time to visit the Bayeux Tapestry (click for website). It records a somewhat earlier invasion on the opposite direction.

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.

Where is the best place to buy the train tickets? Since I'm not sure how long immigration will take, it will be hard to tell what train we should catch. Do they offer tickets where we can go at any time, or or only the time we choose?

Thanks Sam, looks like you posted as I was typing my last reply :)

When I check at https://en.voyages-sncf.com/en/tgv
They don't even list prices for anytime "flexible" tickets for these trains. I'd just buy the tickets when you get to St Lazare station. The return tickets should be bought now since you know you will be in Bayeux for sure on Monday in time to get the train.

RER A and B are operated by the RATP, the Paris Transit Authority, so you have to buy tickets through them. Ticket booths at most stations, or you can buy out of a ticket vending machine.

I would take a cab to St. Lazare for 50 Euro for the 3 of you and buy tickets to Bayeux on arrival and train there. You certainly don't want to risk driving after an all night flight. It doesn't matter if it is slightly cheaper or not; it is not safe. Then return to Paris (you can buy both tickets at the same time) and take the RER A to Marne Le Vallee and Disney. There is no such thing as a zone 5 ticket; trips outside Paris require a ticket from Paris to a particular station. In this case Marne le Vallee. When you are done with Disney take the train back to Paris and your hotel.

If you've driven in France before and don't suffer from jet lag too much, it is possible to rent a car and drive from CDG to Bayeux. But I agree with other posters--with a cranky four-year-old suffering from jet lag,I'd take a taxi to Gare St. Lazare and a train from there. I'd also plan on taking a taxi from the Bayeux station to your hotel/B&B. The full-day tour on Sunday is going to be hard on your child, who will want to sleep, so be prepared.

There are direct trains from St. Lazare to Bayeux, but you can also take a train to Caen and then transfer to a local to Bayeux. Check the SNCF or Deutsche Bahn websites to see what your options are. Gare St. Lazare is a big train station. If you want to use the ticket machines for the train, be sure your credit card has a chip and you know the PIN. It might save you some time waiting in line.