I don’t think we’ll see the Cambridge children in the wedding party.
While we can be almost certain that Beatrice will be maid of honour, her other attendants will most like lily include Maud Windsor, a cousin or niece/nephew from her maternal family, maybe one of Princess Anne’s grandchildren, possibly one of her friends children (Branson’s probably).
If she goes for older attendants you could expect Cressida Bonas/Chelsy Davy (which could be interesting).
As for her tiara, I have almost no doubt that she will wear her mother’s tiara. Also I expect a sleek wedding dress, nothing too big.