They could have used this as a good education project, with Esme researching what designs to do and how many to start with, cost of materials and what would be a reasonable selling price based on that plus time it takes to make, and legal aspects and why these laws are in place. She would then likely come to the conclusion herself that it isn't worth it to sell them.
That wouldn't make them any money though and would involve one of her parents guiding her through how to work out and find this information. At almost 14 and should be starting GCSEs soon she is old enough to understand why it's not as simple as just selling whatever you like for whatever price comes into her head and start to understand that when people want to start any kind of business they don't usually have their parents website ready for them to just add anything to whenever they like.
This is teaching her nothing either academically or about real life, which she desperately needs as when she's an adult she really can't rely on the ever dwindling ifam to love any idea she has.
They say that all the money will be hers, but does she have an account that she has access to, because even Isabelle doesn't seem to. Not really much point in her making money but Chris still the only one that can have it.