Both the venue itself, and the event runners on the day
(which might be somebody who has hired the venue)
can make decisions about who they let in - or remove - or not.
Both those entities have obligations firstly to the safety of children,
then to the safety other people present,
and then, to risk-managing the event so that overall, it runs in an orderly way.
And if that means that they respond to a reasonable request from one patron
to remove another, then (short of discrimination, which this isn't)
yes, they can do that.
Further - no, there is no human rights argument, and no basis for a discrimination claim.
And no, you don't have any kind of rights to go somewhere where you are not welcome,
or where the safety of other children must be a consideration.
And if that means that you don't get to watch kids play sport, then too bad.
Go see your lawyer and get some Orders, before she does.