Adopted children are randomized (got a blonde hair tannish asian looking kid from a pair of white parents), but I don't know if the genes from the adopted parents pass to the grandkids. As far as I know Sims 4 genetics generally focus more on the parents but there is a genuine possibility they could in theory transfer over. Mainly cause I don't think the game differentiates between "adopted" or biological kids. You either have them mix up the genes or randomly generate a sim and slap on the title of "child."
It should be fine, but worst comes to worse you can tweak them to look more like their parents and not grandparents.