Was I unreasonable to give 3yo DD a yogurt that was a week past its use by date?

[EarnestDullard]

I didn't realise till I'd given it to her that the date on it was the 13th. It looked fine, smelled fine, and tasted fine, so I just let her eat it. MIL looked a bit aghast though. WIBU?

You make yoghurt with old yoghurt so there is always a proportion of it that is weeks or months old. As long as there are only yoghurt germs in there, they reach equilibrium and cannot grow any more. If contaminated with anything else it is a different matter. She will be fine.