What to do?

[sjdchbkr]

Here's my problem I will try and keep this short. I found my kids dad about a month ago, He hasn't seen his kids in about 6 years. Just to keep things straight it was my fault, I got married and I let my husband convince me that it was a bad idea for him to be in there life. So any way I found him on facebook, My kids were so excited they couldn't wait to see him. He came to see them 2 weeks ago I kept the 2 kids home for the day so they could spend it with him, everything was going well we went to lunch, flew kites and played video games with them for the whole day. But as soon as school got out and my daughters friends got home she wanted to hang with her friends, she's 13 friends are her world. So she took off my son stayed and spent time with his dad. Here's the problem there dad is dying he was only given a few years to live, I haven't told my kids but I really want both of them to spend as much time with him as possible. What do I do? I'm afraid to tell them he's sick because they might get scared and not want to get close, on the other hand they might want to spend as much time as possible with him. What should I do?