As a teacher, parents always want the other children to be kind but don't mind too much if theirs isn't.
If someone doesn't want to play with their child, because they make them feel uncomfortable, they want them to be kind and include them.
If a child has a party and invites everyone but their child, because they're enforcing boundaries and refusing to invite the child they don't like, they want them to be kind and invite them.
If a group of friends reject someone from the group because they have said or done something the others don't like, or are a bit bossy in games, they want those children to be kind and include their child.
Kindness is short-hand for a lot of things at primary school. Acceptance, inclusivity, compromise, rubbing along with people you're not keen on, biting your tongue to avoid hurt feelings.
It absolutely does not mean accepting mistreatment, accepting bullying, saying yes to things that make you uncomfortable. And at all times, if in doubt about any situation, kids are told to talk to an adult.