Suggested European routes for family touring?

Best (low traffic, fun for kids) route to Harwich: Cambridge, BurySt Ed's, Needham Mkt, Codenham, Woodbridge, Melton, (Sutton Hoo if you need a break), Shottisham, Alderton, Bawdsey, ferry to Old Felixstowe, ride along coast to catch ferry to Harwich.  This is a day ride and limited by Felixstowe to Harwich ferry's last sailing at about 6pm (check this!). Then cycle to Morrison's in Harwich and sit in their cafe. Cycle to Harwich - Hook of Holland ferry about an hour before sailing. This route avoids the A120 HGV hell.

For low cost and great intro to European cycling with family, join Vrienden op de Fiets (a Dutch organisation "friends of the cyclists" - VodF)  an extensive network of folk who offer spare rooms in their houses to cyclists. We've stayed on farms, town houses, house boats, windmills with VoDF hosts,  and made many friends. It's cheap, comfortable accommodation. Meet friendly, like minded locals, get local tips on routes. We can not recommend it highly enough. And in Holland, if any of your group fancy a break, trains are very bike friendly.

harwich ferry then just follow any of the routes in holland following rthe rivers mass 

/rhine, or aim for valkenburg and do the hills around there lovey riding towards aachen over the border 

+1 for the Harwich - Hook of Holland ferry. Cycle routes start at the end of the terminal. Couldn't recommend the Netherlands enough as a destination.

A helpful site here: https://www.hollandcyclingroutes.com/

We've actually been discussing cycling from Regensburg in Germany to Vienna in Austria as a family. The route runs alongside the Danube and is pretty flat. It's mostly dirt road, but in ok condition for the stretches I know. You could rent bikes and I know there are a few firms that offer this. Regensburg is about an hour and a bit to drive north of Munich. You can also get a train now straight from Munich airport to Regensburg, which is a bit like a Bavarian version of York in that it's a small walled Roman city, with a huge studen population. Oh, and it floods sometimes too.

It wouldn't be a door to door trip but it's nice countryside and from Cambridge you're not far from Stansted, which has directs (cheap) flights to Munich.

If you can get to Portsmouth how about sailing in to St Malo and back from Caen?

Le Mont St Michel, Avranches, Granville, Coutances, St Lo, Bayeux, Arromances Les Bains (where the Normandy kandings took place is really worth a visit) and along the coast to Ouistreham to finish.

All fairly flat, wuiet roads and places of interest if that is your thing.  I have only ever done it on roads, a quick look at the Voies verts website shows a couple of routes in that area but I have not used them myself.

You could do, Cambridge - Dover (ferry to Dunkirk/Calais). Head east to Amsterdam, along the coast, past Breskens (another ferry), Middleburg, towards Amsterdam. Then back to Hook of Holland. Ferry to Harwich. Then home I've done the same route, albeit from London.

I've also done Leicester, Cambridge, Harwich, Hook of Holland and towards Luxembourg City. Any Q's, please ask!