Tips   (Now a little outdated)

Cycling
Baskets for children come in various depths, from 35 to 40 cm. In the largest they have the most room for their knees and so can serve longest.
Children's bikes do not excel in smooth running and mostly turn much too fast. If possible you can mount another chain wheel. For longer bike trips, especially in less flat regions the purchase of a 10 speed racing bike is justified. They are already available for 7-years old (22 inch wheel size).
Together we did not train especially, but the children have to be a little familiar with cycling. Teach the small ones to obey immediately when you say "stop", especially in cities they must blindly trust their parents. Also in mountainous areas you have to watch for them as well because they themselves cannot yet estimate their speed quite well. "Brake". "Let go", and instructions when they have to change gear and to which gear, are for the little ones still necessary.

Equipment.
No expensive sportswear for the little one of course, but easy clothes such as T-shirts, shorts with somewhat longer legs (to prevent rubbing), tracksuit and sweater for chilly weather. Of course you can only take a few things with you, so you have to launder en route. Take a basin with you, foldable or it has to fit in a panniers, (fill it up with luggages) is very handy when camping freely. In order not to carry extra shoes we have rubber pedals, also comfortable for children's feet, with special toe-clips in which the foot can rest comfortably. Make one bag the kitchen bag, parafin burner and pans on the bottom, before it the plates, mugs(retractable)and cutlery, rolled in the dishtowel with a few plastic pots for sandwich spread on top of it. If you don't want to restrict yourself with the luggage and you want to use a cycling cart, then this is certainly restrictive on narrow bicycle tracks, bad roads and very exhaustive in hilly terrain.
Camping
A light tent, small, rollable and if possible in several packets is required.
Mostly there is enough room in a 2-person tent for two small children. Air beds are much too heavy. Karrimats give a good insulation and are very light. In summer a down sleeping bag is enough to use as a cover, which means that a parent with one or two children can do with one. Also practical when a child is not clean yet during the night. There are also special children sleeping bags, the bottom of which can be zipped off, which then can be washed quickly and is also quickly dry again. The top is then normal down (Slee Buitensport).
We never camp on a camping site. When you cycle over small roads, you hardly ever encounter one, at least outside the Netherlands. In general it is rather easy to get permission from a farmer to stay for one night. Before you ask, look whether he has a suitable place.
"Look, a complete family" the farmer often said amazed and the children had at once a petting zoo for the rest of the evening.

Routes
In order to cycle nicely and comfortably the route has mainly to be determined by narrow roads with little traffic, even when this means detours. For children traffic is as much as annoying as it is for adults, but it is especially very dangerous, especially with packing they can suddenly swing onto the road. If there are no other possibilities, then cycle diagonally on the left behind the child, so that you fence off the child from the traffic coming from behind you.
So especially with good maps (1:100.000 the Netherlands, 1: 200.000 abroad) to find small roads. Include for the children an amusement park or playground, every now and then a swim, visit a castle, a small town and every now and then a rest day. In short, take the child into account and they will join you next year again with pleasure.