Can't PeterInBrat say something nice...or constructive...or shut up?
There's nothing negative about suggesting a flat commission for a kitchen manager's job. If she's just starting up and only doing $100/day in sales(esp in off season), that's $15/day or about what she's offering now. If a new cook can double sales to $200/day(while keeping cost and quality control), it's a bargain to be paying them $30/day and moreover, fair.
Someone on a salary of less 100 pesos an hour has little incentive to grow the business and work harder while someone on commission does. Every notice how the beach folks hassle you when you walk by? It's annoying, but it sure helps sales. That's what working on a commission does.
This may be news to you, but the DR is not renown for its quality of food or speedy service in the restaurant business. A place may be small, but they can still be doing great sales if they have someone who really knows what they're doing. The last food stand I worked at(US) did $700/day just from 11AM to 3PM and we worked under a 10ft square tent. If the OP could generate that kind of revenue at her storefront with just one other person working, I doubt she'd wince about paying them $100/day.