Never. The biggest reason I am self-employed is so that I can run my business the way I want to and taking on a child with a peanut allergy is not something I would be willing to do. Simply because I don't want the stress of having to do or not do certain things. I just wouldn't want to assume the liability.
That might sound selfish but it's my business. There are plenty of providers out there to choose from that ARE willing.