Hiya, sorry to hear about your mum.
I have been through this situation so totally understand. I’d wait until you find out exactly what is going on, there are many advances in cancer treatment and often it can now be treated so it’s best not to worry them unnecessarily.
If the prognosis isn’t good then honesty is the way to go, your children are old enough to understand the illness and it’s better for them to be prepared not only for the outcome but also for the changes that will happen both because of the illness and the treatments. Your local hospice will have a trained councillor that helps both adults and kids get through this difficult time, they can advise on practical things like how to tell your children and what to expect from telling them.
I hope the prognosis for your mum is good but feel free to pm if you want to chat.
Mark