Yes have gone for the fireflies trip with my family with two children aged 6 and 3. The younger girl was initially scared about the dark and started crying but after like we've gone 5-10 mins into the dark and saw the fireflies, she got so quiet and very fascinated by the insects.
It does look scary I have to tell you cos the boat is quite low and looks like a floating platform with railings. But ended up safe and sound back anyway. I really enjoyed the night scenery, vast night sky opening up before you and the starry starry night.
They have such trips from any of the Desaru hotels, typically RM40 per adult, lesser for children. But if you have your own transport, you can just follow the coach through the plantation till they reach this orang asli like houses. You pay the ticket for RM20 per pax there. Think since the Senai - Desaru highway is up, should be easier to get there.