okay, so if the mother dies (god forbid!) then does immigration stick to the rules very tightly, regarding sponsorship of the children?
Yes, they are very strict. If they weren't medically examined, just assume you won't be able to sponsor them later. (Occasionally people win on appeal on humanitarian and compassionate grounds, but very rarely, and it takes a lot of time and money.)
In any case, there is no reason to not get the medicals (unless the custodial parent refuses). Get the medicals done, list the kids as non-accompanying, then if something bad happens or they want to come when they are older, there will be no problem.
Also in relation to the question, if I waive the right to sponseor them as children can i sponsor them as adults when they reach that age?
If you do not get the medicals now, you won't ever be able to sponsor them. If you get the medicals, but declare them as non-accompanying now, then yes, you could sponsor them up till the age of 22.