If you are a member of a group like The National Autistic Society which provides you with a member ship card or have other proof of your child's disability then you may be able to get free entry to places.
Yesterday, I used my membership card and got into Landmark (Carrbridge) for nothing. Many places don't advertise this concession and I was told that they had problems with people asking for free entry but not being able to provide proof and then complaining! So if you have the proof then ask - you might not get anything but it's certainly worth trying!!