Hi Mich Car - welcome to the forum!
If you want to do everything legally then you should visit the Ayuntamiento at Playa Flamenca and ask them what licences you would require to start your business. If you don't speak good spanish then take someone with you who does as the planning officer doesn't speak english, although one of the admin team helped me out when I was there.
You will need to register with the Social Security system and this will cost approx 250€ per month, regardless of whether you earn a cent or not! Many of us Autonomos (self employed) also pay an accountant rather than risk breaking some laws out of ignorance, so you can add another 50€ a month to that.
Someone on the forum may know specifically about childminding but be prepared for lots of paperwork!
Good luck with your new venture.