I assume you are either driving there or will have a hire car?
A couple of years ago we stayed at a Landal site in Austria called Rehrenberg.
http://www.landal.com/gb-en/holiday-park/austria/salzburgerland/rehrenberg
Salzburg can be done as a day trip if you want to go and see it.
Let me premise this by saying we no longer have small kids so this aspect of this particular site for us wasn’t even looked at. However we have stayed at many other Landal sites over the years when the kids were small and there was plenty for them to do.
The site is just outside Saalbach-Hinterglemm (too far to get the Joker card unfortunately).
There are lots of kid friendly things to do in the area. Look at Saalbach-Hinterglemm, Zell-am-Zee, Kaprun etc.
(Also plenty of MTB stuff if you get the chance)
The Landal sites are very good and have clean accommodation and are usually quite cheap. On cost, assuming you plan to go in school holidays, try and do it as late as possible. I find the last week of UK school holidays is usually very cheap in Europe.