I bought some glow sticks the other day to use them during bath time one night. The ones I bought were not enough, but thankfully I had some more already where I keep all the crafts stuff. You do need a lot to make it brighter.
The glow sticks can be found at your craft store such as Michael's or also at the Dollar Store/Tree.
I hope you do it with your little ones, mine LOVED it and it was probably their longest bath :)
When we finished, they said they wanted to sleep with them, so they both got a couple of sticks and I spread the rest across their room as nightlights. Once they fell asleep I came and turn their actual night light on, just in case the glow sticks lost their glow during the night.