I would have thought that out of the two trades, dressmaking could be the one that would be more likely to succeed. Not only Bridal gowns but gowns for fiestas, comfirmation etc.
With any trade the only way to see if there's a niche in the market is to search the local papers online to see how many people are advertising. It will probably take a couple of years to build up a client base.
Costa Blanca News and Round Town News are the one's I read on-line.
As to schools I think your children are the right age to be able to fit in, the main drawback would be if they were unable to speak Spanish, then they might have to drop down a year until they catch up.