The Dordogne is a wonderful area, we are gutted they have stopped the direct flight there from our home town. I am cutting and pasting an email I sent someone before about the Dordogne, I'll try to review which things would be suitable for a 2.5yr old
Velorail - you would need to ring up and book in French, as for a tourist experience they don't seem to be very organised, but it's great fun
www.veloraildefrance.com/index.php?option=com_content&task=view&id=17&Itemid=38
www.france4families.com/dordogne/RegionsDordogneChateauxCastelnaud.htm
DS loved Castlenaud as its a very authentic castle, plus it has a very nice cafe opposite which does great salads and crepes. Beynac castle is also a fun experience.
www.gouffre-de-padirac.com/
This is a lot better than the website would have you believe as it involves walking down into the deep cave and then going on a boat trip. It is very popular so try to arrive early and do not eat in the closest restaurant like we did as it is overpriced and not very nice. Our friends also did caves with crystals but I can't remember the name of them.
www.mumsnet.com/Talk/general_advice_tips/991463-Can-anyone-recommend-fun-things-to-do-with-children-in
Some great recommendations on this thread, although after having visited Jacquo park I would give it a miss, the water park bit was good, but all the rides seemed a bit rickety and the animals very sad. Canoeing or a boat trip is an excellent way to soak up the scenery although DS did claim to get bored on the boat trip.
www.tripadvisor.co.uk/Restaurant_Review-g425001-d1385843-Reviews-La_Table_d_Erillac-Hautefort_Dordogne_Region_Aquitaine.html
This restaurant is absolutely brilliant if you happen to visit Chateau Hautefort. We stumbled upon it because the first year we went we were staying close. Very child friendly, reasonable prices and amazing food, we didn't find anywhere half as nice in Sarlat to eat although there are some good restaurants with child menus and a few pizza places.
Sarlat itself is lovely in the evening, there is usually entertainment, there was a band and a contortionist when we were there and DS had a brilliant time watching it all, also you can get free samples of all the liqueurs in the shops - do not buy truffle liqueur like my husband as it is foul.
They are very fond of anything relating to duck gizzards or foie gras, sadly I don't like that but managed to find enough to eat.
There is also meant to be riversides that are done up like beaches, but as it was so hot when we were there we didn't make it to one.
There are certain things I would avoid, we made the mistake of going to a museum involving prehistoric digging and bears, it turned out to be someone's back garden that they had excavated and some really sad bears pounding around a dreadful looking area that for some reason he had acquired, we were the only visitors and yet the tour guide refused to adapt the hour long presentation to tailor to the needs of a rather bored 4 year old. There is also the copy of the prehistoric paintings, this may have been fantastic, but I didn't get to see it as they started tutting the minute DS arrived on the tour so DS and I left before he could make any noise.
They don't seem as child friendly in the historic properties as The National Trust and I would keep away from guided tours if possible as they tend to be long and tedious even for adults.
I'm not sure how long it would take to drive to Paris - quite a long time I would imagine. As your DS is only 2.5 it may be one to save for another trip.