Although certain breeds may be more suited to that type of work, I'd have to agree with Karen, it's gonna depend more on the individual dog.
I have 2 Smooth Collies, Oz and Gull and Oz just naturally does the things you're wanting, without any specific training. Whomever he determines is part of his herd, he watches out for them and lets me know if anything's amiss. I don't have any children, but he treats my nephews as if they were his responsibility. He watches them like a hawk and actually gets distressed if they get out of sight. He's very gentle with them. There's no nipping of heels. Collies tend to herd, by nose poking. Although I don't think I've ever seen him even do that towards the kids.
For my other Smoothie, Gull, though, it's all about fun and play. He's not always aware of everything going on around him. He doesn't seem to have that natural watchfulness trait that Oz does. I'm not sure "concern" is something easily trained.
I think it takes a very special dog to be a service dog. Good Luck!
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