I know 2 friends doing pre-school cooking classes and they're really popular, they also run holiday classes/after school classes for older DCs + birthday parties.
One of them goes into nurseries to teach groups there (we didn't have the option, if they went to nursery on the cooking day it cost an extra £3 or so to cover the class) and does it at the school as an after school club, so lots of scope for classes.
She is very proactive with marketing, FB, local radio, local paper/events etc so it does take a lot of time to do it well, but even as a part-time thing it could be successful. The 2 franchises I know of are Crafty Cooks and Mini Pinnys.
FWIW the previous lady gave up her classes because she didn't make much profit after hiring a hall, this one uses her own kitchen for groups of 6 - much more manageable and cheaper. We paid £6 for an hour I think (or was it 2 hours?!) which included all ingredients and a recipe card. While food was cooking kids got to try different foods and as the parent stayed there weren't any issues with allergies etc as it was up to the parent to supervise pre-schoolers.
CC lady catered for a child who had an allergy by buying a specific brand of margarine and the children didn't go near the cooker, they carried their tray into the kitchen and handed it to her. Most recipes were about weighing and mixing, not heating on the hob, but that's where a franchise will help, as they will have tested all the recipes, will know what H&S stuff you need to do etc.
I think its a brilliant idea, go for it.