93p- I'm not an expert, just someone who has taken their kid touring a few times. My top tips
When they are little enough to fall asleep in the trailer/ seat use that day time sleep period to cover some miles. Wrap them up warm tho as the wind chill is high on a bike if your not pedaling. We used to put DS in his all in one winter puddle suit in the trailer. He was cozy and stayed dry even if the trailer cover let a bit of water in on driving wet days.
Be flexible- don't get too hung up on how far you will go each day.
Micro tourism is fun and cheap. Most villages have a play ground,or a church, or a beach or a bridge to play pooh sticks from. Cities have swimming pools, museums and parks.
Once kids are riding solo, cut the miles down for their first tour. AND build them up to it with day rides. We got 6 year old DS up to doing 20 mile day rides before doing 2x 25 mile days touring. Build in rest days, DS 1st solo tour was 2 days riding, 2 days off, 2 days riding. His first tandem tour was 2 riding , 1 off, 2 riding.
National cycle network routes may seem like the roundabout route to an adult, but they can be useful to keep a young rider away from busy roads.
Pack light. We tend to stay in B&Bs or bunk houses these days rather than camp. DS is allowed to bring a very small number of small toys, a note pad and pencil and a bedtime story book and strictly one cuddly only. Cities/ towns have laundrettes and you can wash out cycling clothes on a rest day.
BUT don't forget the waterproofs. If your touring in the UK you will need them. Cycling in full waterproofs isn't ideal, but it stops you getting cold when its lashing it down and you have to ride at the pace of the smallest member of the family.