Also, August is definitely a busy season in the Alps, so that :
<<"Start for Interlaken early in morning. Get a hotel, checkin. ">>
may be difficult.
It would be a good idea to book ahead, and you may find many hotels in the area are already full.
It is certainly true that the Rhine Falls are powerful and large.
They are not very high, however.
And, it is true that the Matterhorn and the Jungfrau are both Swiss Alps.
Both of those alpine regions are different from one another, and the Jungfrau region is probably the most sought after scenic region in Switzerland, and is uniquely Swiss. It covers a large area:
Of course, making choices is required in Switzerland, and what choice you make will be a truly personal decision. Not to make choices, however, can mean not having enough time at any one location to really enjoy it. Further, in the Alps, weather is difficult to predict and can change very quickly. To insure having a good look at or from the Jungfrau, is is wise and common to stay for more than part of one day. Same for Zermatt. Arno's advice is really on the mark. You are setting a very difficult itinerary, even if you did not have any needs for a bit of extra time to travel with a young child.
On average, the clouds tend to come in around Jungfraujoch and the Jungfrau Massif in the afternoon. Thus, on average, it is wisest to plan to go up in the morning, and to check various weather sources right up to the minute before you get on the train at Lauterbrunnen or Grindelwald to go up to Jungfraujoch.
Please check the timetable for your journeys.
Lausanne to Jungfraujoch takes 4 hours and 32 minutes. Further, even if you leave Lausane at 0720 and arrive at Interlaken Ost at 0928, you still have to insert time to find a hotel ( your words) and get to it. At that hour, most hotel rooms that are available are not cleaned and ready. Certainly, you can leave your luggage after you check in, but it takes time. And, the room is unlikely to be available for "freshening up" if you need to. That means that you will arrive at the Jungfraujoch only in the early afternoon. Don't forget some time for lunch. If it is a really nice day in August, you may have to wait for a train seat at Kleine Scheidegg. Or, book a head at the risk that it will not be a clear day, and pay to see the inside of a cloud. It is not a cheap trip. They scenery inside a cloud is not terribly interesting.
Lausanne to Zermattt ( not to one of the alpine viewpoints, just to Zermatt) takes 3 hours; to Gornergrat, for example, almost 4 hours.. Same issues with weather as any other Alpine region.
You can certainly make those trips, and many people go to Switzerland with the specific aim of riding the trains. You did not state that riding the trains is your objective, however. In my personal opinion, you are planning too little time in the target locations to enjoy them, if your objective is to see the destinations in any detail..
I agree with Arno.
Skip Zürich and the Rhine Falls.
Many people enjoyably spend a week or more in and near Luzern or the Jungfrau region. Add time to one of those.
That is just one more personal opinion however. It is your opinion that counts.