Our daycare has that same rule, but the head is a mom herself, and
sees that much of the fun in treats is making them at home, so she
turns a blind eye to small infractions. Maybe what also helps is for
us, the extensive list of off limits foods (milk, eggs, legumes,
nuts, barley, corn, citrus, vanilla....) and my official note in the
office that my children need a special diet consisting of food
brought from home. They have homemade cupcakes and popsicles stored
in the freezer, and I bring rice crackers, chips, cookies (pillsbury
seasonal for girl), veggie fries, (my son can have corn, so fritoes,
corn chips). How about apple sauce?

Is this your own child's birthday? Then they're allowed to bring
their own food. If you let your own child eat it, it should be
hygienically safe for other children. You could also go back to the
'special treat for your own child, and some store bought treat for
the others' idea.
My son will be 5 next week, and daughter 3 two weeks after that- I'll
probably bring in some cookies or mini- cupcakes made at home (those
store- bought ones are too big, anyhow, to small kids- did you ever
watch how much they leave on the plate?)
So I guess my policy is that I blatantly ignore the law (good ol' gen
X- 'it doesn't apply to me' thinking) and IF the inspectors happen to
come by on THAT very day that I brought in the treats at THAT very
time, then we have my note (and a doctors note) on file to back me
(and the school) up.
Ok, now I'm in trouble, I'll just shut up :I
.