No, I personally wouldn't consider overstepping. I do it all the time. Think about this, when parents go to see or do the regular doctor check up, the doctor haven't spend a lot of time with the child to determine (physically everything could seem ok), usually doctors rely on the parents information to check deeper or look for signs. And parents also rely in the carer information (especially first time parents) carers spend a big deal of time with children.

From my view, as a carer you spend hours with the child (giving you more time to observe) and also you have experience.

I do this observations for every child and parents know it. When they go to the doctor I just give them a copy and the pediatricians keep that in their records (ex. First words, physical abilities etc) if there is any concern or sign I just tell the parents what I see during my hours of care but with my list they can also ask their doctor. It is surprising sometimes even parents do know things what their child can do, it obviously is because children spend a lot time with us but also their behaviour is different at home and at childcare.

Also, do not use any labels such as "delays" etc. (No parent wants to hear that). Just nicely give then the check up list (it is available for everyone and actually it helps the doctors); and tell them to ask to the doctor. That's it.

Now it is up to them to follow up or not. But you did your work and "kindly and sensitively" you have shared your concerns. All in the best interest of the child.