it's awful for dd, but school have acted appropriately.
they won't be able to discuss the boy in question (and you should not encourage them to) your focus should be on dd and how they intend to move forward.
it is appropriate for you to put the attack in writing and to copy it to the c of g or to the lea if you are concerned, but it seems fine so far.
the head injury aside (the boy did not hit her on the head deliberately, he pulled the chair out from underneath her and she hit her head) you need to specify in the letter that you understand the school have a safeguarding policy and you would like reassurance that this incident can not recur.
schools do have a terrible time getting additional funding for children with behavioural issues, and one of the things that helps them get this much needed funding is letters from other parents whose children are affected.
if you are up to it, and it is obvious that the other parent was informed who her son had attacked, you might want to offer your support to the other parent in terms of helping her to fight for support for her child (ie a letter to the lea etc). i normally don't think parents should take stuff outside school, but if the school is struggling with the ds, she might be grateful.
it's not uncommon for schools to have to deal with this stuff out of their own resources, but the attacking child is being badly let down here as well as dd and her classmates. doubtless they are looking into this (and how and what isn't your business and they shouldn't tell you) but you need to be assured that some form of support is being put into place.
fwiw, my kids have been in classes where children with quite serious behavioural challenges have mixed levels of support, and the more noise other parents make (about their own kids, not the child in question) the better.
school should have a policy drawn up if this is likely to be ongoing. usually the rest of the class is evacuated to a safe place (hall or next door classroom) whilst the TA runs for help and the teacher deals with the incident/ restrains as necessary (we've had 6 year olds attacking pupils or teachers with scissors and chairs/ throwing tables etc). a good LSA will be able to spot triggers (such as the teacher complimenting another child) and defuse before an incident takes place. but they cost money which is hard for schools to get hold of, and only available in the most severe cases.
hope dd is feeling a bit better x