Viola,
    I hope your daughter is feeling better this afternoon.  I would urge
caution regarding a potential lawsuit. It  can be a very expensive drawn-out
process, with no absolute guarantee of victory.
    You say in your post that your daughter reported the incident to two
staff people yesterday.  Wait a couple of days and give them a chance to
respond.
    Keep in mind that the legal action you are contemplating would
essentially be an effort to get her IEP (a document which you or her father
agreed to) rewritten.   There are several questions which I think you should
consider before you begin the official process.
                    Has she had a full time aide in the past?
                    Does she have a medical history or condition
which makes falling ESPECIALLY                                     DANGEROUS
for her(hydrocephalus, soft                         spot in  skull, etc.)?
                     Have any of the doctors  currently overseeing
her care suggested that she needs constant
monitoring to prevent falls?

    If the answer to these questions is yes, get the relevant documentation
and ask the school to revise her IEP. If it is no, you may have a harder
time getting what you want.

Another aspect of the issue to consider is your daughter's feelings.  Wait a
few days, until the acute pain and trauma are over, and then ask her if she
feels she needs full time assistance.  She may not feel that it is necessary
and unless there are pressing medical reasons to insist, it might be better
to seek out less invasive solutions (such as assistance only in the
lunchroom)  An aide may be necessary and may be able to prevent the majority
of your daughter's falls but remember, it is not a solution completely
without potential negative consequences.  In her response to your post, Joy
mentions the embarrassment your daughter feels when she falls in front of
her classmates. I encourage you to consider the consequences to her peer
relationships, if she goes through the school year with a  constant adult
chaperone. (An aide would, after all, have to be always at her side, if the
aide's job is to prevent falls.)

    Just some issues to think about as you decide what to do next. Hope it
helps!

        Susan