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Well, there's a 99% chance you're thinking of the campfire/children's song "Michael Finnegan," there, but as for live action children's films that feature it, my Google-Fu is drawing a blank. Too much of a glut of children's You Tube programming, and all. Hope that can start your search though.
This is definitely “Wee Sing in Sillyville”, which has come up on this forum a few times! I think this is the song you’re looking for. https://youtu.be/XlzMSBDsZFs
Pre-2005/6 kids show/movie. It opened with live action kids reading a book about a lady. Then they got sucked into the book. The world was pretty pink and live action but kind of fake. She had hair that changed to be in pigtails (not Loonette the clown, but similar). I was a little kid, so I didn’t really follow it that well. She was guiding the kids or trying to get rid of them by getting them back home or dumping them elsewhere, and they had to hide sometimes. There was a bit I really remember where she approached some men who were singing a never ending song where every verse ended with “begin again.”
I was pretty sure this was a dream, but then it turned out Pete’s Dragon was a thing, too, so I figured I’d give it a shot.