The leaf you're talking about very clearly does not have serrated edges between the points. There's a smooth edge and then a point. Then another smooth edge and another point. It's even clearer if you enlarge the picture. That says it's a Norway Maple and not a sycamore.
Like you, I get lots of sycamore seedlings. I also work on a nature reserve where there are sycamore seedlings. I see them at all stages of growth. I can recognise them. I also get Norway Maple seedlings, and I can recognise them too.