T.A.T. is an abbreviation for toxin-antitoxin.
I found that on this list of medical abbreviations:
https://www.tabers.com/tabersonline/..._Abbreviations
I then Googled "T.A.T right thigh" and from the results have deduced that it was some form of injection that caused a reaction in the child. Interesting but complicated (for a lay person) medical sites. I just read between the lines and get the gist of what it says.
Hope this helps a bit.
Pat