If it were me I would call home and tell them that the boy is refusing to eat, which is not normal behaviour for him, and that you are concerned he may be sick as well. Ask what is wrong with his sister. Tell them you had planned to take the kids outside to play but are worried that he is sick too. See what they have to say, since he isn't fully sick then it isn't really grounds for being sent home but a phone call home with your concerns won't hurt anything. At least you'll know what you're dealing with.

I have a family that always send the other sibling when one is sick.