This is a warning to anybody with a swimming pool!!

Based on last year's experiences, be aware that Nivea sun block for kids is EVIL. It has a dye in it to make it "fun". The trouble is that this dye sticks to the pool surround, leaving an indelible green border. It starts off as green bobbles...algae alert but it's not.
Last summer I wrote to Nivea explaining the problem. Apart from an acknowledgement of reciept, they did not reply.
My advice is to ban Nivea for Kids and any copycat products from your pools, explaining to guests that staining will count as damage and loss of deposit

Normal cleaners cannot remove the stain. If anybody knows of something that does, please let me know.

The problem was not just one particular strength of Nivea sun block, but seemed to be all of those labelled as "fun".