As with any business, there is competition in the marketplace. In what seems to me like increasing numbers, some areas are have more childminders than there are parents with children needing to use a childminder. In other areas however there is still a lack of childcare providers.
Having talked with your friend, you have started out on your initial research. Now take that forward and obtain a list of childminders from Children's Information Service and contact as many as you can, pre-armed with a list of questions.
Put yourself in the position of a parent looking for a childminder... with luck that won't be hard to do. Write down the initial questions you need answered. For example, when talking to childminders keep it real - so in your case you are looking for childcare for your 5 year old son and 2 year old daughter. Ask about school pickups - by asking every childminder this question you can plot the data on a map, showing clearly which schools childminders pick up from (and how many childminders for that school) and which schools there are few, or no childminders collecting children.
Think of other questions to ask, hours, cost, which days of the week, typical activities, toddler groups attended, pre-schools collected from (for this one, you say you are considering sending your daughter to a pre-school once she is 3 and thus want to know which pre-school the childminder will collect from. You may get told additional information, such as which pre-school the minder thinks is best).
Talk with the receptionist at your sons school. They can be a mindful of information. They may for instance know of parents who are looking for after-school childcare. They may know all the childminders already collecting from the school.
Build up as much information as you can, so that you have an informed choice with regard to if there is sufficient local demand for what you propose, or not. You may find that you spot a niche in the market, where you are the only person providing the service.