I am very sorry for your situation.
When her time is here she will let you know, and you will know it.
It's as Isiss says "There is just something in the way that they look at you that makes your heart break, but lets you know that the time has come to let them go and that they are ready to leave you".
It's heartbreaking, but you need to enjoy every day you have with her, and not think about when she is not here.
My boy had a limp for about a week and I took him to the vet.
He had a very agressive cancer and they gave him 3 months if they amputated his leg.
I made the decision to give him his wings, and returned home without him.
My girl looked for him immediately and didn't understand where he was ~ they were inseparable.
Over the next couple of weeks she was with me nearly every minute I was home.
I gave her lots of extra cuddles and hugs... and in time I beleive she understood he wouldn't be coming home.
She lived for another 4 years and was very happy and spoiled.
Sorry don't have children, so can't comment on them.
Enjoy every day you have with your precious girls