This could be a herpetic infection though that would mean your children have already had the chicken pox. Herpes is a re-activation of chicken pox virus called herpes zoster. I just came down with the virus and it affected parts of my skin, my eyes and inside my mouth and nose. You'd see tiny blisters in an area on a red base on the skin. Those break open and create a yellow crust and can be very painful. Inside the mouth, the vesicles break open to create an open ulcer. If it gets in the eyes it can cause a lot of destruction and would also be very painful. Herpes could be infectious if your other child got it on her hands, put her finger in her mouth, or open cut. Here''s a link you can look at. https://childrensnational.org/visit/conditions-and-treatments/skin-disorders/herpes-zoster-shingles