With the Easter Hols coming I thought I would add this as not seen it mentioned anywhere else.
If you visit an RSPB centre that has a reception desk you can leave your car keys as a deposit and the kids get free use of the Wildlife Explorer Backpacks. In the back packs there are pens, pencils, track, bird and wildlife spotting cards, binoculars, magnifying glasses and bug jars. Choose the 'family centres' and you'll find your nearest: http://www.rspb.org.uk/reserves/daysout/
It is a great day out and totally free. There are usually loads of things to see and people do not ind if the kids are running around - they just ask they are kept quiet in the hides.
They also do lots of free days at the weekend. Well worth a visit and they are open all year round. Just take wellies if going when it is wet.