We are considering a trip to the Childrens railway although I believe it is 10 miles from the city centre and takes nearly an hour to get there from the City Centre Which is the quickest way to get there from a central pint in Budapest
The 210B bus runs there from the Opera House in under 45 minutes. You can get off near the end and ride the COG Tram 60 if that interests you.
The easiest way to get around Budapest is with Google Maps or the BudapestGO App and, if you are under 65, a TravelCard for the duration of your stay.
Can we use Metro line 2 in one direction to pick up the tram
Sure. If you want. The M2 only goes about a third of the way. Its underground so you won't see the city. Then at Széll Kálmán tér trams stop on several sides of the square, so use BudapestGo to find the stop for Tram 61 in the right direction. Finally, transfer to the 60 Tram (cog). Now it will take you closer to an hour, but it's an interesting trip.
If trains and the like interest you don't miss the Railway Museum not far from city park. And maybe you are in town whe one of the steam trains is making a run?
AND, if riding trams is your thing, I go out to Széll Kálmán tér at least once a week. I take the 4/6 Tram to get there. Interesting ride and I like it more than being underground.