I don't know about extra help at primary level, certainly if your DC go to an English speaking primary but go on to a Welsh speaking secondary they get a crash course in Welsh over the summer holidays (where we live anyway).
I do have a friend who moved to Wales from England, and put her DDs into Welsh school and they picked it up very fast, but they were more Y1/2 age.
I would imagine your Y5 DC would find it hard going from above average in school to below average - they won't be able to write any of their work in Welsh at the same level they would previously been able to write in English. All lessons in Welsh would probably at least to start with could be quite demoralising. But within a few months they should at least be able to speak it well. And Welsh is more phonetical than English in terms of learning to spell...
I guess it depends why you want to put them into Welsh school. Some are very good - there tends to be a general consensus that Welsh schools are good, in the same way that C of E schools are good. But actually on an individual level that isn't always the case.
Do be aware that all their homework instructions etc will be written in Welsh so you may not be able to help them, if you can't speak Welsh you may struggle encouraging them how to improve their written work etc.
BTW I am not trying to put you off Welsh school, If DS1 doesn't get into my first choice of secondary school, he will have a Welsh school down as second choice, because our local English comp is one of the worst schools in Wales.
My 2 younger DSs have picked up loads of incidental Welsh in nursery and reception and they use it naturally in everyday language, they are doing a good job of teaching me too :)