Austria is a very safe place for families. I'n not sure I would do both Salzberg and Vienna though, especially with so many little ones in tote. I would pick one and do it thoroughly. If you are still set on doing Austria and Italy, it doesn't make sense to do a detour to Salzberg. They are one opposite ends (east/west) of Austria, while Italy is South. My suggestion is to go to Vienna, rent a car and drive south through Slovenia, then in to Italy (venice if you like). This way you get to enjoy 3 very different countries all on the same route. Driving in all of these countries is very easy.
My suggested route would be:
1) Arrive Vienna, rent car (parks, museums, "music" galore, zoo)
2) 4 hr drive to Bled, Slovenia (there are caves, hiking, swimming, biking, canoeing, boat rides on Lake Bled)
3) 3 hr drive to Venice, Italy (don't recommend Sat morning though, long lines at border) - venice itself in it's labyrinth layout is great fun for the older ones
I hope it all works out! If you need any other advice, don't hesitate to email
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