@CheshireSplat
Where in North Wales ... ? England may be an option...
Schools in Wales are either "English medium" or "Welsh medium", meaning the school conducts its activities in English or Welsh. So in a Welsh medium school all lessons, break times etc are in Welsh. In an English medium school Welsh will be taught and maybe used a little, but the school day will generally be in English.
Generally speaking, but not exclusively, the further West you go (away from England) the more Welsh medium and fewer English medium schools there are.
There are also private schools which may appeal if your DC are in an international school. I don't know if these are English medium but expect they would be.
This isn't strictly true. We live in an area where all primary schools are Welsh medium, however, my DCs school have mostly English first language children, and so breaktimes etc tend to be in English.. Lessons are Welsh immersion (so all direction given in Welsh) however, every year there will be non Welsh speakers arriving and so the schools are well set up for this. We are an exclusively English speaking home, but DC are fluent Welsh due to school.
Children who enter the primary years halfway through often have additional Welsh lessons to get them up to speed, but at 8 they will pick it up incredibly fast, especially if they are already multi lingual.
When it comes to secondary there are Welsh, mixed and English streams for the children depending on their preferred language. Private schools locally tend to be English language only, with Welsh taught as a second language.
North Wales has a lot to recommend it, beautiful countryside and a generally calm pace of life. Jobs are less plentiful, but if your DH has an offer already you are halfway there.
Where in North Wales are you likely to be based? As this will have a huge impact on how "Welsh" your experience will be.