Maybe I am the odd one out, but here is my take on it:
You have made exceptions (despite saying there are no exceptions) many times for this friend and her daughter. You have printed "warnings" on the forms that have gone unheeded. I'd say, since she is a personal friend, that you need to speak to her privately. Tell her, very straightforwardly, that her constant failure to follow through and turn things in on time with regards to the cheer team is making things very awkward for you. That as much as you would like to be able to help her out, that NO LONGER will you be able to do what you have been doing, which is to unfairly extend deadlines for her daughter and not for others.
Make it plain that this is THE LAST TIME you will do that. She will not get extra reminders or anything that other members of the team don't get, simply because there are too many people and it is unfair to everyone ELSE.
Tell her it pains you to say this to her, and that you hate having to say it, but she has left you no choice. Any event in the future (or order slip or permission slip/form, whatever) she must comply within the time limits everyone else has, or she won't be allowed to participate in whatever it is. With NO exceptions, even for her.
And then, do it.
Thus far, her personal friendship with you has allowed her special status and she hasn't had to follow rules. She is operating under the assumption that those rules and notes on the forms don't apply to her. Really, you BOTH have been operating under that unspoken rule for her.
She will undoubtedly screw up again. But if you give her this personal verbal warning, then it will only happen once. When she sees you actually mean it, she will recognize that it applies to her and her daughter just like everyone else. And she will start making it happen. She WILL get forms in on time. She WILL pay fees on time. She WILL do whatever she is required to do, just like all the other moms. Or her daughter will suffer the natural consequences and won't let it happen again.
It's tough love.
And while the other posters seem to have said not to cut her any slack this time, I think that it is kinda cold to do so cold turkey, when she has been getting away with this all year. It'd be like you changing the rules on her without telling her up front. Because you haven't enforced the rules thus far, with her. Tell her you ARE going to enforce them, from here on out. Period. But this one time, is the LAST time.
Call her up and tell her she missed the deadline, and she needs to bring you money if she wants the Tshirt and for her child to participate. Put it on her. And put her on formal notice---this is the last exception.