I've spent quite a bit of time over the past couple years booking and using accommodation in Europe. Probably the best site I used for booking the type of accommodation you are looking for is hostelworld. It has pretty extensive lists of backpackers, hotels and apartments (which will be really good when you're travelling with a child) which you can book online. For reviews I used trip advisor, but I rarely actually booked through that site because I could usually use find the same place cheaper elsewhere. So reviews on trip advisor but bookings on hostel world (or direct with the accommodation).

My tip would be to use hostels and hotels for shorter stop overs (1-2 nights) but every now and then get an apartment with a kitchen and laundry (or laundry facilities on site). It is great to be able to get all caught up on your washing and cook a few meals yourself when you have been in hotels and hostels for days. I loved going food shopping and finding all sorts of weird and wonderful local foods to cook. Reindeer anyone