We finally got out to try bikepacking with our daughters, aged 1.5 and 4.5.
We did a quick overnight in an area we are familiar with from day rides. We rode out from the parking lot around 5:00, rode for about an hour and set up the tent near a lake. Unfortunately it was to marshy around the edges to swim. I cooked dinner while my wife and girls stayed in the tent to be free from the mosquitoes which were out in force. then they suited up and ate their supper.
In the morning we continued the rest of the loop, with a stop to pick raspberries, back to the car where we had lunch.
We used a Tout Terrain tag-a-long for my 4 year old and a Tout Terrain Singletrailer for my toddler. These have real rear suspension using Rockshox air shocks and so llow us to ride actual mtb trails with the girls, not just atv trails.
In the trailer we had an Ursack with dinners, breakfast, cups, spoons as well as a Jetboil Sol Ti and a small drybag with clothes, these all fit under the seat/her feet. Otherwise we had the tent mesh inner in my backpack, the outer in my wife's and a handlebar bag with clothes/sleeping gear each. We used 2 full length Neoairs and one short one and a size S Thermarest UL. Since it was forecast to be warm we used light fleece blankets. We didn't need all the capacity of the small toptube/handle bar bags we had, so could have left some of those home. Because of the trailers we couldn't use a seatbag, at least not a larger one like in the review by David.
We had plenty of capacity, even for a longer trip or one in colder weather we could have gotten it all in their easily. This set-up still allowed us to ride fairly technical trials at close to normal speed, and our backpacks were light for comfort in the saddle.
We use a Golite Hex 3 which has plenty of room inside the mesh inner for 4 pads next to each other as well as gear around you, the sides of the inner do sag a lot though. We don't use a pole, instead hanging it from a branch or a cord between two trees.
The kids had a blast, as expected, since they love biking and camping. The trail we picked was just right for us, since it doesn't have a lot of climbing, but lots of fun berms and whoop-dee-do's that I and the girls enjoy.