Damn, you got it first. Have a begrudging kudos.

Oh, it being for your new child makes it even better! The shelf being directly at child level will make it so much more fun and engaging, too

I'm guessing your first?

Secure it. Children are nightmarish levels of creative when it comes to choices that are dangerous to them.

A quick example...

My kids like to watch the animals out back. Deer, birds, bobcats, foxes, etc. Upstairs, the windows are a bit lower. Downstairs, they are a bit higher.

My daughter would stand on a chair, or use a stepstool, or even stand on the couch. She's big enough to not need any of those options anymore.

My son also uses the chair, stool, or couch, but he doesnt stop there. The exterior walls are pretty thick, so the interior sill is just shy of 6".

Your guess is correct, he uses the chair, stool, or couch to get onto the sill to look at them. We tried to block access with plants... He moved them or knocked them off. When I put some boards up, he stood on top of the dining room table. He then thought the chandelier made a really neat swing. He is incredibly opportunistic, and will do this when I'm working in my office and mom is cooking or in the bathroom. The moment he realizes he has a solid set of moments, he is off to commit acts of preschool terrorism.

I highly recommend securing it.

Also, congrats!