It's perfectly legal for anybody to sell anything (well, anything that isn't itself illegal) door to door in most places. I'd probably recommend against the candy-bar idea, but that's more for practical reasons than legal ones (buying from a store means you'll not be making much per bar, and any unsold bars will eat into her profits). The bake sale (even a door-to-door bakesale!) idea is great though.
The only thing I'd caution is with regards to asking for donations. To some people, a donation is a very specific term, generally associated with a non-profit organisation or a charity, and after giving the donation, a tax-receipt is generally issued. Obviously, your daughter isn't a registered charity, and won't be able to offer tax-receipts to people who are helping her out. To avoid any complications, it's probably best to make sure that people realise that the money they give her is for her, and not a donation to the church itself.
As an aside, if there are a number of children from the church who will all be trying to raise money for the trip, you might all want to pool together for the fund-raising effort. If you do that, you can probably work out a deal with those candy and chocolate fund-raising companies that schools often work with, and raise more money that way than by buying the candy-bars from stores.
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