If your papoose loves animals and would like to ride a pony, you can make a reservation at the Gavotte's refreshment area to organize the birthday party. Don't wait to be served. Here, parents manage the party. 2 to 3 ponys from La Gavotte (accompanying staff not necessarily available) are reserved for you and the kids to go for rides in the area (one fixed price per poney independent of the number of children). The ponys are super easy to handle but there should be several adults and not too many little ones (ideally 6 to 8). The refreshment area is a rustic but relatively charming wood cottage (in no way modern at all...) et can welcome everybody indoors in case of rain. Drinks can be purchased on location, but there is no pressure to do so. You are responsible for the cake, decoration, dishes, etc.
In addition to the pony rides, children have access to all animals living on the farm. Do keep mind that the farm is still open to the public and not exclusively reserved for the birthday, so keep a close eye on your group before they disperse... There is no set age recommendation but I would say from ages 2 on up is ok, depending on whether your child and his playmates feel at ease with animals or not.