This is a simple but bonkers idea that will just impress the pants off the kids (or perhaps they never wear pants in the bath).
The magic comes from from a few 'magic' everyday items.
The paint is shaving cream mixed with a little Fluorescent non toxic paint. Those blue ones are tonic water ice cubes.
The glowing water- that's Berocca (well, the cheap Aldi version) but anything with vitamin B12 will produce the same glow. The fizzy tablets are fun to chuck in with the kids though.
The key to the glow bath is this uv globe. You can grab these from Bunnings for about $14 and they last for ages. I plug ours in for glow baths and also use it in the cupboard under the stairs for uv painting and other glow fun. They last for ages.