The BEST advice for creatives? Make stuff. Learn stuff. Do both every day.
I have no magic formula or unique words of advice for aspiring illustrators or artists who want to work for themselves.
So, here's the lecture I gave myself:
The best I can say is that to have content to sell, you gotta make it. And to sell it, you gotta learn how. It's all part of a balanced cycle: Learn how to make stuff, make it. Learn how to sell stuff, sell it.
You don't have to go to art school to make art, but you DO have to have enough skill to make stuff, and you do need to develop your level of artistic ability. The best things about art school are that it gives access to and forces you to try different media, some of which people can barely afford (hello Copic markers and Adobe Illustrator-or any "industry standard" Adobe software), it gives you deadlines and feedback, and it forces you to create on a regular basis. But no matter what, you need to keep creating to improve your skills.
You don't have to have a business degree to sell stuff. There's actually a surprising amount of good quality information and business advice on the internet if you look for it, but take everything "experts" say with a grain of salt - what works for one artist does not always work for another. And I am sure I don't need to tell you (but I'm going to anyway) that there a lot of predatory salespeople offering "training" for "the best and fastest way to sell your art", etc.
Whatever you do, don't do too much of one thing or the other. If all you do is make stuff, and you don't learn how to properly market yourself and behave like a professional, no one will know you exist, much less buy anything from you. If all you do is worry about marketing and selling things, your artistic side will go pout in a dark corner and refuse to come out when you are ready to make stuff, and then you won't have anything to sell.
Be artistic, but be realisitc. Not everything you make is amazing or has a market. If you want to make some money off of your talent, you will need to determine your priorities and eyeball where your talents and the paying markets line-up.
Be realistic, but be artistic. Art with no soul is easy to spot, and ambition without hard work is useless. If you always create for the trends, you might make a blip on the radar, but to stay inspired about creating, you sometimes need to make things that aren't "in" but make you happy. (Just don't do it exclusively unless you know there is a market for it. :))
End of lecture.
If you are interested in illustrating for children, I recommend you try the Society of Children's Book Writers and Illustrators as your first stop. It's a good organization for networking and learning the basics if you are just starting out, and has local and regional groups across the USA and internationally.