what was the reason that dolly gave for wanting eppie to be christened?

She points out that it is Silas's duty to baptise his child. She says that the baptism protects the child should anything go wrong.

For if the child ever went anyways wrong, and you hadn't done your part by it, Master Marner—'noculation, and everything to save it from harm—it 'ud be a thorn i' your bed for ever o' this side the grave; and I can't think as it 'ud be easy lying down for anybody when they'd got to another world, if they hadn't done their part by the helpless children as come wi'out their own asking.