Lots of girls who are thought of as tomboys are just girls who don't like all that pink Lilac sparkly princess thing and wearing clothes that are restrictive and delicate and uncomfortable to play in just so you can appeal to the male/ adult gaze.
I was the same as a child, but that's when gendered clothing wasn't such A Thing , so I was allowed to be a girl who just wore red and blue jumpers and trousers and not anything unusual.
My teenaged DD is like this and I'd describe her style as sporty. She wears denim skirts , dungarees, pinafores, skinny high waisted jeans and converse.
For the last wedding, she wore a burgundy coloured playsuit , i think it was from new look 12-15 section, so not expensive. And silver converse and a smartish jersey jacket. And she did up her hair with these little silver clip things ( which apparently don't count as too girly ) and wore some jewellery.
So not traditional wedding outfit but she still looked fine. I'd have preferred sandals rather than converse but hey ho. The most important thing for her is comfort and the most important thing for me is trying not to sweat the small stuff.
Next do lots of styles of kids stuff in many different styles and you can place a click and collect order, go into the store to try it on and return what you don't want immediately . Saves the pain of shopping round with a stroppy non shoppping child.
BTW I wouldn't stress too much about the kids all matching each other, as PP said there will be a range of style in the photo.