[reposting a post I did some time ago]
Macmillan do some really helpful advice on this subject of telling children:
Talking to Children and Teenagers
(Unfortunately, Riprap, the teenager's forum which this refers to, appears to be defunct.)
There's a free Macmillan book available from most hospital cancer centres, and the PDF is available here:
Talking to Children and Teenagers when an Adult has Cancer
This explains that you probably do have to tell children, because they will pick up that they're not being told something, and often imagine it's something else such as parents splitting up, and/or something that's their own fault. It goes through what's age appropriate to say, and how much children of different ages are likely to understand, and ongoing conversations (telling them is not a once-off event).
I certainly appreciate not wanting to tell until you have the answers - that was my position with telling my elderly parents. (Would rather not having to tell them at all, but I am their carer, and it would have been impossible to hide lots of hospital appointments/treatments. Same considerations would have applied as trying not to tell children.)
Wishing you the best with this.