For travel from Milan to Paris there are faster options of course, but slower travel through Switzerland is a much better choice. Especially if snowplay is the goal, as that is sure to be available at the Jungfraujoch. We went up with my tween nephews on a railing day in August, when it was snowing at the Jungfraujoch, and they (bing Californians) had a great time, especially with the platter-sliding run.
I do have some suggestions for shortening the train time and hopefully making a better trip for your children. I also suggest that the parents each get a Visitor Half Fare card for 120 CHF, and one of you get the Free FAmily Card that can be included. This will make all the train, boat, and lift rides free for the kids.
May 15—train to Luzern on the Treno Gottardo (take local train from Milan to Bellinzona to meet this train, IR 26, at 9:59 am, or 11:59 if you prefer. You will arrive in Luzern in less that 3 hours, with no changes on the way, and have the afternoon to explore the town or take a boat ride on the lake.
Alternatively, you could ride the IR 26 train as far as Flüelen on the lake, and catch a boat there to reach Luzern. We did this (in reverse) last September with our 11-year-old twin grandchildren, and both they and their parents loved the trip, both the scenic train and the boat. There is a small playground between the train and the boat dock if the boat departure does not line up with the train arrival.
May 16: Golden Pass train to Interlaken Ost, transfer there to the local train to Lauterbrunnen, and then the little train up to Wengen. This car-free village is a much better place for children than Interlaken, and also it is partway up to the Jungfraujoch so shortens your journey the next day.
Book 2 nights in Wengen. I suggest the Hotel Bären, which is family-friendly and has a playground adjacent. They have their Family Room available for these 2 nights (305 CHF with breakfast) but you will have to check about adding a baby cot for the infant.
https://www.baeren-wengen.ch/en/hotel/playground-playroom/
May 17: Jungfraujoch. Some people will advise that this is weather-dependent, and you shouldn’t go if it is cloudy or raining, but for your purposes it really doesn’t matter, in my opinion. You are not going just for the views, and rain at Wengen should be snow falling at the higher elevation. Make sure the kids are dressed for the cold and wet, and they should have plenty of fun outside. They will also enjoy the sculptures in the Ice Palace. And crowds should be way down, especially if the weather looks bad.
May 18: To Paris.
