Don't forget the lake.
We just had a 4 year old and a 18 month old visiting for a few days.
If you have access to a boat, the 4 year old may enjoy slow tubing (with mom or dad), "driving" the boat (on captain's knee), or learning to cast and catch fish both from boat/dock/shore. The 18 month old enjoyed watching all of the above.
Not sure if anyone mentioned it, but there is a family circus setup at Faro's at the Weirs - have not been, but it may be entertaining for the young ones.