I say it's all about the benefit vs the detriment to the patient. If you think seeing the grandkids will help improve his mood and add a sunny spark to his day, then I say bring her. If you believe that your daughter and her cousins will be more exhausting and stressful for him, then I say leave her home.
You do have to check with the hospital visiting policy as well, and find out what floor or section of the hospital he is in also.
My mom had cancer and eventually had a bone marrow transplant. My daughter, who was about 4 was able to visit her everywhere, even in the bone marrow ward - she did have to wear a mask there though. It was good for my mom and it was fine for my daughter. Yes, hospitals are very germy places, but these days they have hand sanitizer everywhere. Do not bring your daughter if she has the slightest hint of a cold as this isn't fair to your FIL or any of the other patients, but otherwise go. Yes, she will climb all over the lounge furniture if you go out to a sitting area and you will cringe the whole time. Bring some wipes and wipe her hands periodically.
On the other hand, if he is not in for an extended period, just do what you feel is best for your daughter. If he'll be out in less than a week, don't even bother bringing her - you could always have her draw a picture for him or make a get well card and bring that.
Best wishes to you and your family.