I do not believe they are. Hydrangea has opposite leaves and it appears as though the leaves showing are alternate.

It looks like a Mulberry seedling.

I would have to disagree and say it IS a hydrangea, based upon the two HUGE hydrangea I have growing.

Here is a link that might be useful: Compare

I grow several H. macrophylla, H. arborescens, and H. paniculata, as well as a couple of H. petiolaris (vines) and your leaves don't look like any of them.

I'd go for Mulberry seedling too.

Mulberry seedling. They come up in droves whenever there is a mulberry even within a block or two! Birds spread the seeds.