Here is one idea. I have not checked it against the timetable. You can reduce your confusion by looking at a map and the timetable. It is not the only possibility, but is the first thing that comes to mind.
<<"Can you please edit the below itineraryry which you think is better......
D1 -31 July Landing at 10AM at Zurich then off to Lucerne">>
Will you be jet lagged?
How much energy will you have to explore Luzern on Day 1? Will your child be exhausted from the travel? Time zone change? or, are you coming from Europe? If exhausted, consider taking lake boat ride.
D2 -1 August -All day in and near Luzern, boat ride and cograil to the top of Rigi or to Pilatus. Perhaps explore the city thoroughly, depending on what you did on Day 1. Possibly go to Interlaken in the late afternoon via Golden Pass Route (Luzern to Interlaken Express.)
D3 -2 Aug Lucerne - Interlaken by Golden Pass line early in the day, or, if already in Interlaken,explore some of Wengen/Männlichen, or go up to Schilthorn, include easy walking in the area. Details available with further discussion. No long walks due to 3 year old.
<"D3 -2 Aug Interlaken - Jungfrau
D4 - 3 Aug Interlaken - Zermatt
D5 - Aug Zermatt- St Moritz on Glacier Express
D6 - Aug St Moritz to Zurich by 6 PM">
To see the scenery from the Jungfrau requires clear weather, or else you spend a lot of money and look at the inside of clouds. The weather changes frequently and suddenly in the Alps and the detailed forecasts are only good for about a day ahead of time.
So, don't get your tickets to the Jungfrau until the last minute. Consider adjusting your arrival time in Interlaken based on consultation with the tourist info by track #1 in Luzern.
Consider exchanging day 2 and day 3, based on weather - also stay in Luzern on day 2 or go to Interlaken based on forecast for the Jungfrau .
That is what I would use a starting plan. If I had time on my trip to Zermatt on Day 4, I would definitely take the higher and very scenic route via Kandersteg and Brig, rather than the faster and boring route through the new, lower "basis tunnel" from near Spiez to Visp. Check the time table.