I sent an email to Panadol yesterday:

I just went to use our children's panadol, which I keep in a zip lock bag in the baby bag. It had spilt in the bag, so I thought I must have not screwed the cap on properly. But on inspection, the cap doesnt click in to be child proof at all and is actually quite loose. I just thought I would let you know as this could be quite dangerous.
they called me today & said they were changing the lids, so I'm guessing this is happening a bit. Definately worth checking, especially those of you with older children who could open them...