Fly to Salzburg if you can - it is only 60-90 minutes from there to many Austrian mountain resorts, either by public transport or, as there are 5 of you, by private taxi. Munich is also fine if that is where you can most easily get to - 3 hours by train will get you to most places.
Many places offer summer "Tourist Cards" that give free use (or reduced price use) of cable cars, local buses, outdoor pools etc. See for example the Joker Card offered by Saalbach-Hinterglemm.
Mayrhofen and Zell am See both have veteran steam train services in the summer.
None of the places you've listed are particularly "built up" or "developed" - obviously there are hotels, restaurants and activities, but once out on the mountains there is plenty of peace and quiet.
Accommodation ranges from simple but clean B&Bs (called "Pensions" in Austria), self-catering apartments through to all-inclusive 5-star hotels with pools, "wellness" spas etc. - and every level in between. There are also hotels that cater particularly for families with children, and which are marketed under the general term "Kinderhotel" - although these are all independently owned and operated. See for example the Ellmauhof in Hinterglemm for the sort of things these offer such as playgrounds, play rooms, nanny services, petting zoos and ponies etc.