You can book it now to secure the venue. I think it dones't matter how long in advance you should book it (off course at least 1-2 weeks) provided that no one book your favourite time.
Last year, my son had a party at McDonald (there is no Mac Cafe). Around 80 people (including adults), the bill was around $2,000. I know, if there is a Mac Cafe and people order food from Mac, the sum will be higher.