This picture helped to connect my Sarah with the Sarah in the 1890 picture. One year around St. Patrick's Day, the children were showed the pictures of their Irish ancestors. When Dad wouldn't let the kids take the original pictures to school, my daughter asked if she could draw their picture. While genealogical vacations are difficult with a spouse, they are even more problematic with small children. See if there are any historical spots near where you will be doing research.
Children can also look at tombstones, noticing the different types of stones and engravings.
Care should be taken with small children at cemeteries, however. Old stones, on unstable mountings, have been known to topple and in at least one instance a child was killed by a tombstone as it fell to the ground. Make copies for the kids to look at. You can even discuss how the ancestor made his or her letters. This can be especially interesting if the child has a relative who has the same first name as the child.