I recently took a trip with two children (4, 18 months). We did a sleeper train from Brussels to Prague (EuroSleeper) and from Vienna to Zurich (NightJet operated by OBB)

The OBB trip was phenomenal. Great amenities, and we were on their older model. So I can’t imagine how great the newer carriages are.

The EuroSleeper was awful. Bad amenities, never received our (prepaid) breakfast, temperature was stifling (85° F, 29° C) and the ride was not very smooth.

I would gladly do a sleeper train again, but maybe not with kids so young. Probably not until the kids are 10+