Not a childminder but do know of one in this area who charges extra for occasional hours which is what she would class the above as...those with regular number of hours per week pay much less. She tends to charge by the hour and the rate is quite high to reflect the fact that you are 'fitting the children in' and could be losing another regular child because of this. May be worth considering in this case...dont know exactly how she works it but she does offer childminding at short notice to mums if she has room and is in fact very popular because of this.