I actually would organize activities, and I also would ask someone (in advance) to help me with this (or hire a HS/College student)
By age 5, I'm assuming the guests (I mean the little 'uns) are in school and understand concepts of games and organized activities. Personally, *I* wouldn't let them just get hyped up on sugar and run wild. but that's me.
It doesn't have to be overlly rigid, but there should be *some* organization:
1-1.30 arrival, greet the guest, and let them do some kind of art work (decorate a crown, decorate inexpensive pair of flip flops which will be their 'goody bag", etc). (have music on in the background). supervisor: Mary.
1.30-2.30: games. Pin the wings on the tinkerbell, duck duck goose, musical chairs, whatever you like. just take into account that 12 kids (and you may have more) may be a lot for a game where they have to wait. if you consider that each attempt at "pin the wings" will take 1-2 minutes, that's 12-24 minutes that they will be sitting around doing nothing. you may reconsider or have two tinkerbells.
supervisors: mary and Lilacgirl
2.30-3.30: refreshments, cake, birthday wishes, gifts (if you are opening gifts). I would start with the actual food and end with the cake.
supervisors: mary, lilac girl and...
after that - let them play with yard toys or sit them down to watch a cartoon (depending on how hyped up they are) or both