Can I switch from a tourist visa to a Non-O visa for Thai child guardianship while in Thailand?

Yes, it is possible to apply for an in-country initial Non-O visa based on a parent of a Thai child, if your local immigration office allows in-country visa application. You also need to have the minimum remaining days with your entry stamp to apply for an initial visa.

Go to the local immigration office for their latest requirements for an in-country Non-O visa for that reason. Usually, the funds don't have to be seasoned for the initial visa application for Non-O based on a Thai child.