I put myself first and now I regret it

I (28M) am a single dad of twins (3F). We'll call them Alexa and Siri. I co parent with my daughter's mom (35F) and we split 50/50 custody. We have a great co parenting relationship and rarely run into any issues. I recently started therapy and it has been doing wonders for my mental health and dealing with some issues I've been having for a long time. Recently, their mom got sick with the flu on one of her days and the following day post exchange, Alexa got a fever. I gave her some medication and I took her to work with me and let Siri go to daycare that day as she was not sick. I usually have therapy every other week and didn't want to miss it. Their mother offered to take Alexa for an hour or so assuring me she had gotten some medication and was starting to feel better. I was hesitant, but agreed and dropped her off and went to my session. When I picked her up she was fine, but later that night her fever came back and Siri had a fever as well. My mom offered to watch them so I wouldn't have to miss work, but their fevers got high and I had to take them to the urgent care. Siri only had a cold, but Alexa had an ear infection AND the flu. I'm upset because I should have just canceled my appointment and not let Alexa go with her mom. I strongly feel like she got the flu from her. Now all of us are sick plus my mom and her husband. This could have been mitigated if I had just canceled that damn appointment, and I feel terrible about it.