Yeah you made a few OC sakura and Shirou kids right mike?
One of them had shadow swords right?
Well due to the hair colour think it seems like you don't believe in lamarkian genetic inheritance but both shirou's sword powers and sakura's shadow powers were kind of gained via modification and not their original genetic stance. Also aren't the powers a kid gets pretty random in Nasuverse lore beyond crest inhertance. So my only advice to you is try not to draw too much from the parent's power set unless you use some crest stuff. If it is a newly founded crest those things are kind of terribly so if you do have inhertance via crest the abilities they should be pretty nerfed as it takes multiple generations for a magic crest to get any good.
The swords are not due to an origin or anything, he is just good at creating things in general (not by projection, he uses ether), and fighting like his father is a natural approach. As for the shadow powers, no, Sakura's shadow magic is actually her natural alignment, it's not something Zouken pushed onto her. And, yes, Sakura's kids all have a crest she created for them. It's not amazing, but it allows them to use Sakura's magic, because that is what a Crest
does (it stores copies of the spells that person created, and spells created by all previous family members who had added to the Crest).
I will admit that IIRC (I'd have to check the sheet) I have given at least one of the kids shadow as part of their elemental affinity, but the inheritance of elemental affinities is not clear. Some families (the Matou, for example) do have a fixed elemental affinity, others seem not to. So, there is a fair amount of flexibility there, I feel. And, neither of the kids have binding as their family trait, they are both fundamentally Tohsaka mages.
Also in regards to "her natural color", Sakura's body was changed on a genetic level by Zouken. For that reason, in procreational biology, her "natural" hair color would (now) be considered purple.
Where does it say that?
Biology simply does not work like that, you cannot change a person's genetics and have them survive, plus there is no such thing as "Matou genetics". Plus, as Gray said, eggs are made prior to birth.
So, no, sorry, but I'm going with "they're Tohsakas rather than Matous".