GAP (Great Allegheny Passage) is an excellent choice with kids. Beautiful scenery, rail to trail, easy camping, lots to see and do. You can easily extend on the C&O tow path towards DC. Wife and I road the GAP and C&O w/ 3, 5, 10, and 12 year olds, more or less self contained. 3 year old on recumbent tagalong and 5 year old on trailabike tagalong. Both outfitted with 2 panniers. 10 and 12 year olds rode own bikes w/ front and rear racks. 3 matching back packing tents, sleeping bags, water, food, etc.

Very nice first tour w/ kids.
Highly recommended.

https://www.gaptrail.org/