Why aren't Kisa and Aunt Tiger planning to escape?

Aunt Tiger wants to assist her friends and neighbors before leaving herself.

I now have many people whom I love and most of all, I am able to help many people. I've found new purpose in life. This simple woman is going to do some good. I now know so many people who want to escape to the South. I want to help arrange a few more trips for the neighbors and then I will leave. It won't take too long."

Kisa refused to escape because it would put his family in danger.

Kisa sighed. "I can't go. If I disappear, the Russians will notice immediately and will send a search party after me. Then we'd all get caught and be brought back here to face the machine guns. Aunt Tiger and I have to be here to cover for you and give you a chance to cross the border. The two of us have been too visible. You must leave now and we will follow soon after. I have to run.