Looks to me as though you should not worry about a pass, but buy point to point tickets OR get a Jungfraubahn pass which will cover the immediate area around Wengen. Passes are really only worth it if you plan to do a lot of train travel, but you really have to sit down and figure it out step by step -
List all the train journeys you will take and then check each one on the SBB site for the cost and write this down. Then add it all up and if a pass works out cheaper, get the pass.
For the Jungfaubahn info
Note that the Jungfraubahn covers trains from Interlaken on so if you choose to get a round trip ticket Zürich airport to Interlaken Ost and a Jungfraubahn pass from then on your journey will be covered. If on the other hand you are going to wait till the day after you arrive in Wengen to start your local travels, you will need a ticket Zürich airport to Wengen.
If you are going to hike then taking the child in a backpack will be a lot easier than pushing a stroller. But on the trains it doesn't really matter... most of the newer trains are low level entry. And if there are steps there are only a couple.
However you need to be careful about how high you take the child. Doctors here tell us t^hat children under 2 should not be taken above 2000m. Wengen will be fine - it is about 1200m. Mürren will be OK too, but the Jungfraujoch and the Schilthorn should be avoided. Reason is the lower level of oxygen in the air as you go up.
Finally be sure to bring warm clothing as well as summer things. it can (and does) snow in the mountains any month of the year and having a couple of sweaters, gloves and a hat as well as a really waterproof anorak and good quality and waterproof hikinh boots with wool socks is important. Hopefully we will have a lovely warm summer but you never know and it can be cold in the mountains in the summer...