There's a clear communication issue here. If the childminder was worried that your ds wasn't drinking enough during the day, she could have kept a record of what he drank one day, and discussed it with you - and then you would both have had some facts to look at. You would have been able to see whether or not your ds was drinking enough, and if he wasn't, then you could have discussed different strategies for coping with this.
I would discuss with her how the issue was handled, and say that in the future, you want better communication about problems like this.
You do need to remember, though, that the childminder might have to use her own judgement sometimes, on matters that you have laid down guidelines on. Issues may crop up whilst she's caring for your ds, and she might have to make an on-the-spot decision, and you need to decide whether or not you trust her judgement.
In my opinion, giving your child some weak squash because she was worried that he wasn't drinking enough one day was her exercising reasonable judgement - but where her judgement failed was in not discussing it with you that day, so that you could look at alternative ways to make sure your son got enough to drink.
As someone has already said, very weak squash (without aspartame) isn't going to harm your son or his teeth - especially if he stops drinking it from a sippy cup.
Are you otherwise happy with the childminder? Is your ds happy and settled there? If so, I really think you would be foolish to disrupt your child and his childcare over such a small issue. Rest assured, there are going to be so many small issues you will come up against during your child's life, in his friendships, schooling, childcare etc, and if your first response is to haul him away from whatever it is, and move him somewhere new, then you are going to have a very stressful time, and so is he.