Just had a great overnight with my two youngest kids. It was a short out and back, about 2.5 miles each day. My son is 9 and my daughter is 11. It was the second time I have taken my son and the first trip for my daughter. The night was cold at 35 degrees but we had warm enough bags and an extra blanket.
I don't have much experience backpacking myself so for me the key is to keep it simple. We went close to home, just a two hour drive to the Manistee National Forest. A short hike, only about 2.5 or 3 miles each day. The terrain was easy but there was natural beauty to keep them interested. We walked past three lakes and camped near a lake. We saw a pair of Bald Eagles on the hike out. Most importantly both kids felt like they accomplished something.