The most charming children's hike I've ever seen is the Muggestutz Trail, which is in the Berner Oberland, but not near the Jungfrau Mountain. It's so special, though, that it's worth the time to get there.
It has signs (in German and English) along the way that tell a story about a fictional dwarf who lived in the woods on that mountain, and which explain the different elaborate children's activity stations representing parts of the story. It is through woods and is only downhill. It has spectacular views of the Rosenlaui glacier-topped mountain and the mountain range that leads to the Jungfrau. It has a picnic table area with campfire stations which are well supplied with kindling wood each day. People bring hot dogs called "servalot" to cook over a campfire with their children. Your son is the perfect age for it.
Here are informative links with some pictures:
The Muggestutz Trail is on the Hasliberg (Hasli Mountain) east of Lake Brienz, and would take about 2 1/2 hours to get there from Wengen, where I see, from your other posting, that you plan to stay (although if you stay in Lauterbrunnen, instead, you could get there a half hour quicker). You would take the train from Wengen down to Lauterbrunnen, then change to the train that goes to Interlaken Ost, then take a very scenic train ride around Lake Brienz to Meiringen. At Meiringen you walk from the train station about 15 minutes to the cable car station--it has a nice playground (ask directions at the Meiringen train station), and take cable cars up to Mägisalp where the trail starts. It ends at Bidmi where you take the cable cars back down to Meiringen.
A very nice playground with a gorgeous view of the Jungfrau Mountain and mountain range is a cable car ride up above Wengen, at Männlichen. It even has a child-size primitive bowling alley made from logs. To get down a little hill there, you have a choice of the stairs or a slide. That is near the beginning of the hike to Kleine Scheidegg, which is only 2 1/2 miles long and has benches along the way, with a spectacular view. I've hiked it 7 times, including twice this summer, and it was well maintained and easy except for one year when I was there the first day it opened for the summer (June?) and it still had snow patches melting. So I make sure to only hike there in the heart of the summer. It has restrooms at the beginning and the end. A train at Kleine Scheidegg takes you back down to Wengen and on to Lauterbrunnen. I saw a playground in Wengen, too, but I don't recall anything special about that one.
The cutest playground I ever saw, and also with a stunning beautiful view, is across the Lauterbrunnen Valley, above Mürren at Allmendhubel. That's the Flower Park that some other posters already mentioned. To get there from Wengen, you would take a train from Wengen down to Lauterbrunnen. At Lauterbrunnen take the cable car up to Grütschalp. At Grütschalp take the train to Mürren. At Mürren walk over to the funicular and take it up to Allmendhubel.
By the way, trains from the Zurich airport that go directly to either Luzern or to Bern have a playground in them! The train carriage with the teddy bear symbol on the outside is where you find it, usually at the beginning or end of the train. It's on the second floor.