Like bigboots, my dd goes to a Welsh medium school. She's already (age 7) well past my reading competence in Welsh.
I can speak a little, understand spoken Welsh reasonably well, but I can't read beyond about say ORT level 4 or 5 (not helped by the fact that Welsh is a very geographically diverse language, and what I understand is Pembrokeshire spoken Welsh, which often bears little resemblence to written North Walian).
I would say at least 3/4 of the children in dd's class have parents who speak as little or less Welsh than I do. This doesn't seem to stop them all doing perfectly well in school.
Indeed in areas where there is a choice (unlike here), middle class English speaking parents notoriously opt for Welsh medium education because it is considered higher quality & better resourced - I've heard it described as 'private education paid for by the state'!
Similarly plenty of English first language children opt for Welsh medium secondary education here, even though they have the choice of an English medium secondary, because they prefer the 'feel' of the Welsh medium school (or because their best friend is going there, or whatever).
The only disadvantage I would see, potentially, in sending dd to a school was if 90% of the children spoke the same non-school language, because I suspect this would become the language of the playground, and the English speakers could feel isolated - and less likely to pick up the majority language if it is not also taught in school. I do know Welsh speaking parents who avoid dd's school for exactly this reason, because the playground language is generally English. But if children are coming from many different language backgrounds, then English will almost certainly be the dominant social language.