oooh yeah i forgot about that. The sefer Minhag Yisroel Torah (I feel like I got that name wrong its a few volume set on so many minhagim and their mekoros) says that he looked for a mekor for sechel and gelt and couldn’t find any anywhere. He asked many gedolim and they didn’t know either. But he saw that they did it anyway since “minhag Yisroel Torah”.
What’s actually brought down in halachah is to “wash your eyes with the wine” to show chavivus hamitzvah. Maybe people didn’t want to burn there eyes out from the alcohol content and put it on their eyelids and their kids thought they were putting it on their foreheads etc.
I’ve also heard that there’s a minhag to put on the place where the kesher of the teffilin shel rosh is.