Hello Heidi
Really sorry to hear about your son's accident - when they are so young, and don't really understand what's happened it's even more upsetting for child and parents alike, especially when there is a lot of blood involved. I hope that he's feeling a bit better now.
It certainly appears that you may have good grounds for a claim. Basically, the play area should be safe for users and, whilst rough and tumble is part and parcel of play, you would not expect there to be dangerously exposed surfaces that can cause injury if fallen against. The play centre operator has a duty of care to ensure that the equipment is safe - and it sounds like they may have been negligent by not having the pole padded.
You've done all the right things - and hopefully you also reported it to the operator at the time and there is an entry in the accident book (which they should maintain for such incidents) - by going back and taking photographs. Of course there is also the medical record from the hospital too.
As your son has suffered facial scarring which will be permanent it will be regarded as a serious injury. Because your son is only 5 years old he will need what is called a 'Litigation Friend' to pursue a case on his behalf; usually it will be a parent. All this means is that the Litigation Friend will sign all of the paperwork and deal with the solicitor. There is a limitation on personal injury cases of 3 years to bring a claim, or if it involves a minor 3 years from them attaining the age of 18.......so you have plenty of time!
Good luck
Steve