We're starting our trip in Munich and then leaving for Chamonix but we have a couple days in between. We know we'd like to show our 8 year old Neuschwanstein. We thought about seeing Salzburg but it's in the wrong direction. Is it worth a night or are there other cities (e.g. Annecy, Lyon, or some city in Switzerland) that would be good substitutes.
If your child has seen "Sound of Music" (or if you watch it later), Salzburg will look familiar. It's a beautiful small city. To a child, one city is about the same as the others, but because of the movie, Salzburg might stand out in their memory. Just a thought.
I don't know if there are any good substitutes, because I'm just starting to be able to take travel adventures. I have been to Salzburg, and am going back this spring. I really liked it, and want to go back whenever I can. We didn't do the touristy things before, just walked around and soaked it in. This time, we'll do some. But I think it's worth the time.
I would skip Neuschwanstein before I'd skip Salzburg. Neuschwanstein is an icon and should probably be seen once but it's nothing compared to Salzburg. Salzburg has a real castle, great churches, gardens, etc. - and all very walkable. I've been to Neuschwanstein once. It's ticked off my list and I won't plan to return. Salzburg is one of the most beautiful cities I've seen and I always try to visit when I'm close. In that regard, it's like Venice.
Salzburg is a fun city and easy to get around. Our son was little when he was there, but the fortress, the gardens around Mirabelle (where Do Re Mi was filmed), and the trick fountains at Hellbrunn were big hits. Have him read up a bit on Mozart, too.
Salzburg is a wonderful place but it is in the wrong direction as you said. There are plenty of great sights between Munich and Switzerland. Neuschwanstein is not a fortress, like that in Salzburg, but it is a beautifully situated palace. I am not sure which would impress your 8 year old more. The lakes and foothills of the Alps in the region are are also very nice. You might look into the north shore of Lake Constance (Bodensee), perhaps Meersburg. I really enjoyed Lucerne (Luzern): lake and the town. The area with its mountains and attractive villages is as picturesque as any place. None of these places are a substitute for Salzburg but rather wonderful in their own right and more along your route.
I know it's the wrong way, but Salzburg is so close and easy from Munich. It shouldn't be missed. Kids would probably enjoy the castle, fountains, gardens, and zoo, but parents dig Salzburg for the beauty of the city, this history, the culture, and the music. Unless I'm mistaken (and Lee can correct me), you can use the Bayern ticket for Salzburg...28€ for all of you Mon-Sun for one day 9a – 3a the next day. Salzburg is a UNESCO Heritage Site for a reason...try to fit it in, even if it's just as a daytrip. My only recommended substitute is Vienna, but that's not going to work for you.
Sorry, there is no substitute for Salzburg (or as the previous poster said, Venice). They are both unique. There are alternatives, but no substitute. Salzburg is genuine. Nobody yet has mentioned Berchtesgaden or the salt mine. I was in my mid 50s when we went down the banister and we still have our photo! Gluck Auf!
I hate to say it, but I would skimp on Munich and see Salzburg and Berchtesgaden/Eagles Nest. If your 8 year old is well behaved and not afraid of heights, Eagles Nest can be a very breathtaking view, if the clouds are not heavy the day you go. You can really see the inner circle of Munich with a half day walking tour. To really explore Munich, or any city, you would need much more time, but that would be my trade-off. Fuessen and the castles are a great visit, and if you drive, you can see Weis Church, Linderhof, and some great scenery (aka Lake Plansee, and others). The decorated/painted houses in the area would probably thrill your 8 yo too.