That is good advice above. I also take my kids 3,5 and 7 to Landsdown in Clinton, like the D&R but shorter and less crowded. It leads to the coopalong which is slightly more like singletrack, ice cream shops are in town as well.
My kids, especially the 5 year old love high bridge pump track.
I live behind echo hill park in Clinton, it has short but nice entry level trails for kids, some hills they may need to walk but they are short, there is playground and pond so you can mix it up with play and fishing. It is small, so not a destination for just biking. you can also piece together trails from the south branch park and they will feel like they are doing real mountain biking.
I plan on scouting out the French town preserve, which would be perfect if doable for them, wife and 3 year old can tear up the canal path and we can do the preserve.