1
Ask them not to rub it.
2
Sit them down facing the light, and gently open their eyelids to look.
3
If you can see it on the surface, rinse it out by pouring clean water from a glass or jug, from the inner corner of the eye.
4
If that doesn't work, try lifting it off with damp gauze or the damp corner of a clean handkerchief.
5
If it won't come out, or it's very painful, they should call 111 or go to an eye casualty or urgent treatment centre.