I think it's a brilliant idea, as a tax practitioner
See, the majority of domestic employees are paid cash in hand. So by giving the employers a tax break, it costs the government virtually nothing. Nada. However, what happens is that the employees are traceable and taxable, and the government raises money by getting tax it otherwise would not have got.
Unfortunately, although it would be nice to extend this to childcare, this would actually cost the government big time, because nannies mostly declare tax, and so do nurseries and childminders.
So I can see the tax logic in this.