Paris hotel with toddler?

you might look at the aparthotels which have some one bedroom i.e. two room suites. They also have kitchenettes which is useful with a toddler.

What a fun adventure! Check out the Hotel des Grands Hommes and its sister hotel, the Hotel les Dames du Pantheon. I know that the Grands Hommes has a suite, I do not know if Dames does (however it has the possibility of connecting rooms). We have stayed in both, and they are wonderful boutique hotels. Both have a lift, and both offer breakfast (which can be brought to your room on a tray - just tell them the night before what time you would like it). The location is excellent - right on Place du Pantheon (pay a little extra for a balcony or view if you can, and if it works with your room configuration - it’s worth it!). You will be able to easily walk to Notre Dame. You may need transport to the Arc (depending on how well that stroller “walks”!). I think either of these hotels would tick a lot of boxes for you. Wherever you end up, have a wonderful time!

https://www.hoteldesgrandshommes.com/

https://www.hotellesdamesdupantheon.com/

This doesn't answer your question, but Notre Dame is closed from the fire. You can take him to La Sainte Chapelle, which is open but has very, very heavy security for the next nine months. The Bataclan terrorist attack trial opened in an adjoining building yesterday. Or go to Saint Germain church, which is very colorfully painted inside.

But you can still see Notre Dame from a short distance or from across the river on the south side.

Will have the stroller, so ideally near a metro stop with a lift,

Sorry to say but you will find hardly any (none?) metros with step-free access. I think most RER stations have lifts, but of course in the city center they are far fewer between.

We stayed here with kids in a suite/connecting rooms, and it was a great location:

https://www.grandhoteldupalaisroyal.com/en/

Also, when my kids were toddlers, I traveled with a kids hiking backpack rather than a stroller. Was much easier to get around and my kids liked being high up. We had one similar to this:
https://www.rei.com/product/126512/kelty-journey-perfectfit-child-carrier?CAWELAID=120217890004997992&CAGPSPN=pla&CAAGID=107685713500&CATCI=pla-303037211915&cm_mmc=PLA_Google%7C21700000001700551_1265120001%7C92700056635456820%7CNB%7C71700000062011463&gclid=Cj0KCQjw4eaJBhDMARIsANhrQAB2LYHEbdsrV2MO51YzujClOz4i9cfRMfTcsJ42RY1yWZk8mVj5kRcaAusIEALw_wcB&gclsrc=aw.ds

You probably know this, but most metro stations have lots of steps not just a few to enter the station. And if they have escalators, they probably don't allow strollers. Depending on how much your child weighs, the child backpack idea may be good but the good ones are not cheap.

Try a junior suite here
https://www.hotelresidencedesartsparis.com/

Great location and a separate bedroom.

I would only take him to places that interest you and are predominantly outside. He will enjoy the moment, but it is highly unlikely that as a toddler he will have a lasting memory of what you see. He is more likely to remember, "fun with mom" than "I saw Notre Dame."

I think the open top bus tour could be great if it is something that you want to do. Once again, it will be "Mom and I rode the bus" not "we saw main tourist sites."

I think the walk from L'Arc de Triomphe to the Trocadero would we a good walk for the two of you.

Busses will be much easier with a stroller than the mêtro. You enter at the back with a stroller.