Hi there Anniken64,
I believe that Thessaloniki has a lot to offer to its visitors, no matter what the age is.
Regarding the spas that you mentioned, I have tried my self the one at "Egnatia Palace Hotel" in the city center. It's a relatively small one, at the basement of the hotel, but I was totally satisfied from their services. You could try that one.
Moreover, Magic Park could be a very nice break for your child, but if the rain starts pouring, then in a walking distance of a minute or two, from "Magic Park", you'll find the biggest mall of the city, which is called "Mediterannean Cosmos", with many different activities for kids and not only. There are many cinemas there, bowling and games in general, many restaurants and tavernas, cafes, many many shops.
The Planetarium which is again very very close to Mediterannean Cosmos, could also be a great idea!
Now, if the weather is nice, the "new" waterfront of Thessaloniki is very very nice. You can walk from the White tower, or take the small train from there, and you'll be able to see on the waterfront the new parks of the city, and maybe have a snack and a coffee or something.
As far as museums are concerned, there are tons of them for every kind. Archaeological, Byzantine, the Macedonian struggle museum, contemporary art museum, and many many many others.
Please feel free to ask for more details if you need anything.
Friendly regards,
Yannis.