This is only preschool.
This is your child, so naturally you worry.
Fine.
But... per friends and your child: ever since my kids were Toddlers (I'm talking from 2 years old), I spoke to them about friends... and how to discern "choosing" friends. Not that I expected them to choose friends to my liking or not... but it was to "teach" them, about how to "analyze" people in general. My kids are now 5 and 9... and they are very good at choosing friends and discerning people/kids/other parents in general. I also taught them how to speak up. And they are not "followers" or copy-cats, per other kids. Even if they do hang out with them. They also know what is right and wrong.
So, the point is: in addition to how our kids make friends or with whom, YOU also have to... teach and guide a child on people and social situations and how to "think" about it.
I am very honest with my kids, without bad-mouthing others. About others. They are now wise, about "analyzing" social situations and the "intentions" of friends.
And, there will ALWAYS be other kids, that may gravitate, toward our own child, to play with. It just happens. So, again, if you teach your child how to analyze and discern other people, your child will have "skills" on how to manage.... situations with others.
There is for example, a girl in my daughter's class... that is a known Bully and inappropriate. This girl, "likes" my daughter. That doesn't mean my daughter is her friend. But, my daughter is darn well aware... of the situation... and how this girl is. So, my daughter is able to manage... her interactions with this girl or not. SHE.... is not just sucked into... the behavior of that other girl. SHE... can limit her interactions with her and whatever dialogue occurs between the two of them. AND, my daughter tells me, what goes on in school, per this girl.
THAT is also, what you want to encourage in your child... that they can TELL you things, about others or other kids/parents... that they may see or what they see happens. At school.
..I, do not have to worry... that other kids are going to influence my kids... or that my kids are push-overs. Why? Because, I have taught them... all about discerning people, and how to speak up... since they were very young.
Also, just because your child is a friend with some kid, that you do not like the parents... it does NOT mean, you "have to" have play dates with that family/child. It can just be kept, as an IN school friend. You dont' have to socialize outside of school.