Is Apple Motion the right tool to make fun videos with my infant?

I would start with iMovie. It's free.

Motion is not exactly a video editor. It can be used as such, but it would be a pain.

Motion is great for designing motion graphics.

It is an amazing application, and I enjoy it very much, but I don't think it is the tool for the job, plus it has a much steeper learning curve.

Hardly indicated for a beginner, I'd say.

iMovie is very easy to use and includes a number of prebuilt animations that are ready for you.

If at some point you start feeling you have outgrown what it can do, then I'd look at FCP. FCP is far more powerful, but harder for a beginner.

Anyway, since there is a free trial of FCP, you could experiment with both FCP and iMovie and see how you feel about each.

iMovie is the go-to answer, and it's OK. I've taught middle school and high school level video classes over the last two decades, and I always found it a bit tricky to get folks to think that way. It's good, but I'd not say it's fantastic. Nothing I've seen for free is fantastic.

Also look at CapCut and HitFilm, colleagues of mine have used these in middle and high school video classes and like them.

If you want to do that sort of "compositing" effects, you'll want Motion (takes some learning, but isn't bad). But titles and such are available in both iMovie and FCP. Star with iMovie. Once you're comfortable with it, try the free trial version of FCPX and take an online tutorial from macProVideo or Ripple Training. FCPX gets way more sophisticated and some formal basic training will save you tons of time and headaches.