Firstly - congratulations! Secondly, don't wait to tell your head. They need to put in place measures immediately to ensure your safety. If it usually takes two of you to restrain this violent child then it isn't fair to put all that responsibility on your TA, nor to run any risks yourself. I would also question walking into an SMT meeting and announcing your news. Do you want lots of staff knowing immediately? I also don't think it's the right forum, I would find it quite bizarre if a member of staff walked into an SMT meeting to announce that they were pregnant. A quiet word with your head and/or line manager would be better. Good luck with your pregnancy and do remember that your safety and that of the baby must come first from now on.