Hi Zohra -
<<" staying back in zurich for sightseeing after keeping my luggage at the airport lockers">>
There are arguments for either option - going straight to Wilderswil, or, doing a bit of sightseeing in Zürich.
The flexibility of the Swiss travel system will help you with your decision. You will have lots of choices of trains:
If you have any of the standard rail passes or cards, you can travel on any train. Even if you don't have a pass or card, reservations are not needed within Switzerland, so you can leave your luggage for a while and explore, or go straight on to Wilderswil
There are about 6 trains per hour from Zürich Airport to Zürich Main Station (Hauptbahnhof or Zürich Hbf.) They take from 10 to 20 minutes, depending on number of stops. At your arrival time, some will be full of commuters, but some of them are special trains that only run to and from the airport.
There are a variety of "Day Cards" for the trams in Zürich City . Or, your Swiss Travel Pass will cover the costs.
There is also a "Fast Luggage " service whose details are available on the SBB website. It might be possible to deposit your luggage at the ZURICH MAIN STATION before 1100, and pick it up after 1800 and before 1830 luggage office closing at Interlaken Ost that evening, maybe not at Wilderswil. For a hefty fee.
If I were dong it myself, I would get my bags at the airport, leave customs, walk out and across the street to the Airport Services+Train Station Building, and go directly to the food court in front of you on the street level. (Toilets are on the far far right.). The luggage carts available near the carousels are free, and they can go up and down escalators with a heavy load. You can take them all the way to the train.
(Stay back one step when the escalator moves the handle of the cart up into your chest or face as the belt with steps tips downward.)
If it were I, I would get a cup of tea or coffee, assess my/family's state of tiredness or eagerness, perhaps get a bite to eat from the many kinds of food services, chose whether to stop in Zürich oity or go direct to IO/Wilderswil and then go downstairs to the train station. It is a couple of levels down. If you have your Swiss Travel Pass, I think that you need to validate it the first day that you use it. That may consist of a date stamp from a machine, or writng in a date. Check the links above.)
There are four tracks available at the lowest level. One pair goes toward Zürich main station. Not sure which tracks serve the trains that go to bern witout going to Zürich.
Go down and catch which ever one you want. The doubledecker cars have flat floor level with the platform. Easier for heavy luggage.
Most trains to the Berner Oberland will bypass Zürich Main Station. Some will go to Zürich Hbf before continuing (or changing to ) a train for Interlaken. Bern is a normal train change on many routes; it may be listed as a destination for intermediate trains..
If you choose to stop in Zürich for a while, leave your luggage on the mezzanine level just below and beyond the rail heads. This level has lockers ( up to 9 CHF for the larger ones), toilets, and some airport services.. If the lockers are all full ,go to Left Luggage, which is is on the outside wall of the terminal, to the left as you leave the railheads on the main level. (Gepäckaufsgabe).
There are plenty of restaurants and takeaway's in the station. The COOP across the street has a takeaway section.
I like to wander around the old city near the Lindenhof. And, both sides of the river near there.
Their is a friendly Tourist Info office for Zürich in the main station.
I think it has just been relocated to the lower level in a new section....ask.
After you have spent anywhere from 1/2 hour to a few hours in Zürich, you can catch a train for Wilderswil via Bern and Interlaken ( I or W) at : xx02 and xx32 ( every half hour, all day long, taking a bit over two hours. Usually two changes sometimes three, including one at Interlaken. Hotel might pick you up?
If I were doing it, I'd stop in Zürich and take a quick look around, since there are such frequent trains available whenever you get tired..
During the day, there is a restaurant car on the main stages of the ride to Interlaken.