Nursery workers should be paid by their employers during bank holidays as part of the business running costs and not from charging parents.
The running costs come from charging parents. Particularly with funding for 15/30 hours being below the cost of provision, fees pay for the business running costs.
Most nurseries will be like mine. They charge for 49/50 weeks a year (incorporating bank holiday and Christmas closures in the other 2 or 3 weeks) and then average out over 12 months. So you’re not paying more in January than in February, etc.
Bank holidays are built into that fee structure. To have a further rebate for bank holidays your fees would have to go up (and I’d expect Mondays to become more expensive as they would incur additional expenses for the nursery).