Hi
I also have a sports mad ITP son, although he is a bit older at 11. We've also had a lot of problems with getting him to accept that there are limitations in sport and activity generally.
I think a lot of the guidance would depend on how liable your child was to bleeding. However, for what it is worth, we have been told the following by the consultant
a) absolutely no heading the ball in football unless his count is high - never been told exactly what this is but I reckon 70-80 at least
b) No contact sports unless his count is over 30
c) no playground footie unless his count is over 30 - because it is on tarmac and unsupervised
d) no competitive football unless his count is over 30 - this might not matter so much for a 5 year old!
e) no "off-the-ground" activity unless his count is over 30 - climbing frames etc
He has however continued to play football in the playground against instructions and not come to much harm. He has also continued to go to football practice with his team, and he absolutely loves tag rugby which is classified as non-contact. Your son is just the right age for this because they don't do real rugby until they get older.
Its hard to stop someone doing something they love, but I think you have to when their counts are really low. If my son's count is over 30 though, I let him get on with things - the doctor says its important not to stop him doing things if he is ok, and the worrying is my problem not his! I have found it helps to provide something special to do instead if his activity is restricted. When he hasn't been able to play footie for example, I've made a special effort to book the leisure centre and some of his friends for table tennis or badminton or something
Hope this helps, Ali