I agree, at the very maximum, a glow bracelet could be used to pacify the child's fear of the dark and minimize the amount of light disturbing other riders.
However, being a parent of a 4 year old and a 2 year old myself, I probably would err on the side of caution as to which dark rides to go on with them.
Last September, my 4 year old handled HM beautifully, in part to us telling her that it was a "silly scary" ride, and we'd whisper in her ear what was coming up next. We gave her the option of wanting to ride after we went through the stretch room, and she did great. I would have never considered taking a small light on for her, that's just me.
We took her on Winnie the Pooh and Peter pan first to test how she'd do. We "worked" our way up to the other stuff.