Either Scotland Ireland or Holland around 20-40 miles a day.
Any suggestions?

no where is as cycle friendly as holland, if by family you mean younger kids, Id go holland as it surely is much safer / traffic free

Outer Hebrides.
Great beaches. Quiet roads. Friendly drivers (according to our bike-hire customers).

Holland. Ferry to Rotterdam Europort or Hook. Day round Rotterdam, loads of sights, amazing architecture. Riverbus up to Kinderdijk to see all the windmills, they take bikes on the boats and trains. Down to Dordrecht, nice youth hostel in the national park. Up to Utrecht via Gouda to see the cheese museum. Utrecht hostel in a Napoleonic mansion. Up through Amsterdam to the coast beyond for a stay in a castle at Heemskerk. Back down either the coast or inland through Haarlem to Delft or the Hague, and back to the boat.

Cheers for suggestions will looking to both Hebrides and Holland routes. Kids 10+7