I agree with Paul and Karin on this. Remember exercise can effect the immune system both positive and negatively. If you do too much you can deplete the immune system making it more likely they are sick again. Go easy with them. No interval training/HITT.
Stick with low intensity cardio, use the RPE scale and Talk Test and do circuit training for resistance. Start with at most 30 min exercise and no more. Increase the time of exercise before the intensity of exercise.