Like @fossyant, my little one was just in own brand Halfords lids when she was little, they're fine enough. And their heads grow so quickly It's not worth overthinking it, get one that fits and is cheap, it'll be too small next year and you'll be buying a new one anyway. She has a POC helmet now she's older, but her head is pretty much adult sized now so each helmet lasts much longer.
Yes, deeply so that this is what was commented on from my post. It deflects from the important message I was keen to give regarding safety buckles in the light of a local tragedy where a girl died in the most appalling circumstances because her helmet was not so fitted.