Any old rucksack will do, we started with a nice bear one from Next but we soon broke the tubing detail due to needing too much in it. Big enough for least 2 full out changes, coat, waterproof wear and wellie ideally.
Just a kids rucksack. We got ours from Amazon. I've done too much safeguarding training to get anything with a name on the outside and thus far too paranoid.
My sons just got a kids rucksack