The requirement for children throughout 5 years of residence under the GV is that they are in full-time education, are not married and are financially dependent on the main applicant. It is not a requirement that they are under 18.
My kids are currently 18 and 20 and we just completed our biometrics last week. By the time we start our 5-year clock they will in all likelihood be 19 and 21. Add 5 years to that and they will be 24 and 26 by the time we have completed 5 years of residence. Fortunately they are in university and will be looking to go on to postgraduate qualifications after their undergraduate degrees, which are 4 years long in both cases.
I don’t know what the requirements for permanent residency after 5 years of GV are, whether it is automatic or if the dependent children still need to be in full-time education, single, financially dependent. I very much doubt the latter though - are they supposed to remain dependent forever?
So I think so long as your kids remain in FT education throughout the 5 years of residence - and they don’t get married and also you can demonstrate some ongoing financial support of them - then you should be ok from that aspect.