I think The Netherlands (and Belgium) are a great choice for touring. We’ve recently done our second trip there with kids (now aged 11 and 9) taking the ferry to Rotterdam riding down the coast and then through Belgium via Antwerp, Brussels, Ghent and Brugge, returning from Zeebrugge.
I’m used to riding in the hills, but I don’t regard Holland as bland cycling. The cycle routes have lots of variety. If you go down the coast route, the path winds along the coastal dunes through lovely woodland, and takes you through pleasant wee towns and villages. You also get to cycle some impressive long causeways over the tidal barriers. There are miles of long sandy beaches, which you can cycle right up to. Lots of interesting diversions en-route; we stumbled on The Bunker Trail, a network of German bunkers that have been excavated and restored. The Belgian network is not quite as good, but still great cycling. There are good routes along the major rivers and ship canals, with free ferries for pedestrians and cyclists to hop from one bank to the other.
We were camping (towing the tent and gear on a trailer) and there are lots of campsites, but there is also the Vrienden op de Fiets network, a network of people who do cheap B&B, but only for people travelling on foot or by bicycle. One of my friends has used this and said it was a great system.
You can cover reasonable distances without too much trouble because the terrain is so flat. The routes are well signposted, and they are all covered by Google Maps, so it is dead easy to plan or adjust your route on the phone. It’s also easy to hop on a train with your bike if you need to cover bigger distances.
The thing I enjoy most about the cycling is how relaxing it is. You can roll straight off the ferry onto hundreds of miles of car free paths, which is great with kids, and also quite enjoyable as an adult.