Holland has, except for hiking and biking a lot of nice things for your kids. Maybe some of these things do not really fit well into a biking and hiking holidays, but it may help you to keep keep your kids happy.
Bicycle riding for kids
Just a few words about cycling. Dutch kids learn to ride a bike when they are 2-3 years old. It's quite normal to see 4 years old kids go on bike to school. Please, don't try to compete with this. If your kids are not used to ride a bike, take time to practice and make them wear a safety-helmet.
Because of its good infrastructure Holland is a very safe country for kids to use a bicycle. Kids from the age of 7 can do a bicycle holiday in the Netherlands, provided they had some training. There are many separate bike lanes and level crossings with other traffic are well secured. Car drivers are used to cyclists and will behave rather careful normally. But, occasionally you will need to cross a busy highway with high speed traffic.
Always nice for kids is to visit a zoo. There are about 10 zoo's in Holland, too many to mention them all. My favourites:
Rotterdam, Blijdorp zoo. Extensive and they made a lot of progress in creating natural looking areas for the animals. A nice playground for the younger kids. You can reach it easily by tram frm the backside of Rotterdam Central railway station.
Rhenen, "Ouwehands dierenpark". Not very big but they have the brown bear wood, where they rescue circus bears and give them a a new living. You walk over a wooden bridge above the bear wood. Pure joy to see happy bears. It's on walking distance from Rhenen railway station, which you can reach over Utrecht. Website for the zoo.
Apendoorn, Apenheul. This zoo focusses on monkeys in all sizes. You need to pick up a monkey proof bag for your food, because you can be directly in touch with the smaller monkeys. You are allowed to feed them at some hours with food provided by the guards. You can reach Apenheul fro Apeldoorn railway station with bus nr 3 every 30 minutes. See also Apenheul web site.
Harderwijk, Dolfinarium. Dolfinarium is specialized in sea animals, with Dolphins as their major attraction. If the weather is good, take you bathing clothes with you. There is a a nice sand beach!. For me the top attraction are the fish you can feed. They eat from your hand!. You can reach it by bus from Harderwijk railway station (at the line Amersfoort-Zwolle, stopping service).
Marine museum Rotterdam. There is a musuem building, but I like it much better to walk along the water and see the historic vessels. Some of them are open for a visit. It's on walking distance from the Rotterdam Central railway station (if you like hiking) but you can also use public transport to reach the museum.
National Railway museum, Utrecht. This museum is very attractive for kids. It has a large number of historical railway stuff, stored in the old "Maliebaan"raiway station. Younger kids can make a ride on a miniature train. The look and feel is not like a traditional museum, but more like a kind of tivoli parc. You can reach it from the Utrecht central railway station. Sometimes there is a train going stratght to the museum, but you need a lot of patience. I normally walk through the old town of Utrecht to the museum.
Speelklok museum, Utrecht. this museum in the old centre of Utrecht shows a large number of mechanical musical instruments. You really need to take a guided tour for the demonstrations of the beautiful street organs, mechanical pianos etcetera. On walking distance from Utrecht Central railway station.
Zuiderzeemuseum, Enkhuizen. An open air museum with focus on the sea side buildings around the former Zuiderzee (now called Ijsselmeer). Demonstrations of old professions.. You reach it through the town of Enkhuizen or by boat form the railway station. Enkhuizen.
Open air museum Arnhem. I think the first open air museum in the Nehterlands. Preserves old agricultural an industrial buildings from all over the country. You reach it by bus from Arnhem railway station.
Holland in miniature, Madurodam in The Hague. Madurodam is a collection of scale models of Dutch buildings. You can see things like the golden coach of the queen, the peacle palace, wind mills, a railway station etc. Combine this with a visit to Scheveningen beach. You can reach Madurodam with tram no 9 from The Hague Central station.
Fairy tale park Efteling. The fairy tale wood is at the heart of this Tivoli park. I think its better than Disney World with attractions for kids of all ages. It does not have a web site in English. You can reach the park from 's-Hertogenbosch or Tilburg railway station. Bus 136, 137, 168 will bring you there.