I'm going to Paris with my wife and my 3 yo daughter. We plan to buy a convertible bicycle/ stroller trailer.
Is it OK to ride the Metro while taking your kid along in a trailer like this?
Everything is possible. Strollers are allowed in the metro.
But I would advise against it because there are too many stairs in the Paris metro that could make your rides far from enjoyable.
If you read french, here is a blog post on the difficulties of a young mother with a stroller: R.A.T.P : Rame Avec Ta Poussette ! The article is relatively old, but situation has not evolved since.