First of all, let's just get it on the table: When you take your 8-month-old trick-or-treating, you're not fooling anybody. I don't care how cute her little doggie costume is, everybody knows the candy's going to be for you. So unless you're actually willing to let your infant gum her way through a pile of Snickers and Tootsie Rolls, just let the farce go. You're a big girl--go to Wal-Mart and get yourself a nice big bag of candy and just eat it all yourself. You can even toss it into a plastic pumpkin first if it makes you happy.
Secondly, it is never okay to ask if you can hold a stranger's baby.* It's just not. Especially not while you're trick-or-treating and your judgment has already been called into question for the reason described above. Not even if you say, "I have a baby too!" What sort of ridiculous reason is that for me to let you hold my child? During flu season?! Go hold your own baby, then, and quit pushing her around in a stroller while you beg candy from your neighbors.
*I suppose, in the interest of full disclosure, that there are SOME instances in which it is okay to hold a stranger's baby. Like if somehow you witness someone toss their baby into the air as they fall down three flights of stairs--I think in that case they would be glad for you to catch their child. You might not even need to ask.