thick toenails??

uconnhuskie007

So my DD was born 5 years ago with really thick toenails (kinda gross, actually) i asked her pedi then who said "she's fine" every well check i show him and he insists its ok. she says they dont hurt but they look brittle (when i clip them they break into pieces) and a couple of them have a black streak. He insists shes ok but i feel that it's weird and unusual and i hope it's not a sign of some health issue.  Anyone know what this might be, or have experienced the same?