The Kitchen currently does a "family style" breakfast only on the weekends. They do not offer everything they originally had, but still have a lot to offer. Mimosas are not bottomless, but they are $9.50 for one, or two for $15. For the family style breakfast it's $23 for an adult (do not recall price for child, but want to estimate it was $18?).
Unless it is a holiday I do not think you would need a reservation. However, you can always call to double check. As most Universal restaurants it's not a requirement, but for The Palm in the HRH if you want to dine with them for dinner it's reservation only. The Kitchen you can walk in.
Also, while I'd love for you to have breakfast at Universal, I would not give up on Disney just quite yet. Reservations have to be cancelled within 24hrs, or the guest will be charged a fee. It would be one of the higher fees as well due to it being Topolino‘s at Riviera. As the fee is based off what kind status the dining is. People often cancel within just in time before the deadline. It's the reason why I was able to snag Be Our Guest for dinner only the day before a trip of mine once. You just sit there and keep hitting refresh on the website. So, that being said, if it's something you do truly want and want to try I would start checking within 24hrs of when you'd want to go. You can start looking the morning before the date you're wanting Brunch, or may even try looking the night before that just out of curiosity to see if you could snag it sooner.
