Looks like something my LO experienced when she was an infant. Started off as ringworm around her neck, then when it recovered, her skin looked discoloured, now her skin tone is back to normal. So I would think thats their new skin... Continue to moisturise your LO’s neck.
My LO had a small patch discolouration when he was newborn. PD prescribed Zaricort cream to apply. It went off after a few applications. you may wanna try that.