I’ve run several online stores over the years. All very simple to set up and run, but generally either not making any money once you factor in expenses, or end up completely taking over the house and every evening of your life!! One of the businesses grew into £250k turnover with 4 employees but then a big change in the market meant it collapsed very quickly and we were lucky to get out of it selling the stock on to someone else. So my advice would be to grow naturally, start as soon as you can even if that’s only with one or two products, to prove the concept and start building a customer base.
If you’re selling stuff for little ones you’ll need to make sure it’s all CE safety checked.
Most important thing is keeping good records (something I’m shit at!) from the start, so get some easy accounting software/app etc and make sure to log every penny you spend and receive. That way you can be 100% transparent for tax purposes.
Your pricing needs to reflect not only the cost price for the item, but also your overheads, your time to pick, package and post, plus profit. The idea being that you should be able to pay someone else to do your bit in future and still make money from it, so profit is NOT the bit you pay yourself, but the part on top of all the other costs that you should be able to invest back into the business in future.
Maybe start out selling on local FB grips and get a page up on there too, and see what demand is like. Might also be a good time of year to go to a few Xmas fairs.