She has the flu

My eight year old daughter has a sore throat and cough that started about 4 days ago. I have been giving her medicine but it doesn't seem to be helping. The cough is worse at night. We tested her for covid twice and both tests were negative. So I assumed it was a cold. My baby (under 1) developed a high fever today so I took her to the ER. She was diagnosed with Influenza A. She was prescribed Tamaflu (?) I think thats what its called? We were discharged pretty late so we are picking it up in the morning. Anyways, since I'm sure she got sick because of my oldest, is it safe to say my oldest has the flu? Should I take her to get tested? I'd probably have to wait a few days unless we go back to the ER but she hasn't had a fever. Anyways, can I treat her with dayquil or mucinex or should I wait to get her tested and get her the medication my baby was prescribed? We've never dealt with the flu before. I'm a little scared for them.