There is a difference between sewing for yourself and sewing for others. When someone chooses to buy a handcrafted item, they do it because they want something unique and stylish. Anyone who would consider any of the products you suggested would expect perfection. In all reality, as simple as diapers and slings seem to be, if you expect to entice customers to buy your product, you're going to have to be more creative, cuter, more user friendly, and cleverly designed than what's already available in stores now.
Start looking at mass produced and name brand products as well as, going to websites like Etsy or local high end boutiques that feature handmade baby products produced by local cottage businesses. That's your competition. If you can't come up with something on par or better (such as a fit unmatched to the others out there, or some unique design or fabric), perhaps find another plan.
Secondly, if your product does take off, are you skilled and equipped enough to mass produce your product and still maintain quality? I think just knowing and/or learning how to sew isn't your biggest concern. You won't last long if you can't fulfill orders. You could get hundreds of orders in a day. Would you be able to make enough of these items to keep up? Do you have a set up at home or a place that you can make into a studio dedicated strictly for making these items, interference free? Realistically, a half an hour or hour or two a day while the kids nap isn't enough time to make anything well...even for a skilled sewer. So a dedicated space is a must as well as dedicated time. Where would you store items until they are purchased? Where will you store preproduction supplies? Who will handle accounting and orders? How do you plan to ship etc. Take in considering how long it could take you to create and perfect your signature product, and how to get it to the customer in good efficient time, maybe this isn't a good, let alone financially feasible endeavor unless you are perhaps able to get a small business loan and can make accommodations/delegate duties to others to help get these tasks done.
Short of starting a sweatshop in your garage staffed by illegal aliens, or hiring or finding fellow friends and skilled sewers who would be willing to get a small stipend until you get on your feet and can pay them more, chances are you won't realistically be able to meet production goals and maintain quality all on your own. You might be able to make a few items and sell them here and there, but then you will not make a profit. Then this would really be a hobby.
Last, as others have mentioned. Besides basic lessons, you really will need to take additional classes on more specialized sewing skills such as pattern making, tailoring, embroidery and knowing how to use common industry machines such as a serger (really a must for speed, certain stitches and fabrics and is just a must as it is a time saver), cover stitch (needed for finishing), and embroidery machine (for monogramming, personalizing, unique details and finishing touches). You will also need to know how to use the other important tools of the trade, such as quality irons and steamers for fabrics, good scissors (can cost alot), rotary wheels, sewing tables so you will have the ability to adjust patterns (in case you have special orders or sizes), dressforms in various sizes (especially so you can size for the slings) and so on, so you can really make a very polished product.
People want detail and personalization, not basics. They will expect you to tailor things to their exact specifications and needs because you are not mass producing these. If a mom with an unusually large baby needs a super large diaper in a size that exists no where, will you be able to alter your patterns to fit her needs?
Furthermore, If you don't know how to use machines like those mentioned above (there is an art to it for sure) your pieces will not look professional. People want special if they go with a handcrafted item. Otherwise, they will just buy it from the store. You will find, you will have to charge alot to break even, especially in the beginning. So if they have to pay alot, it better look and be worth it. If you don't have alot of money to begin with, this might be tougher than you think also...sewing IS expensive, even with coupons etc. As I said, when you're sewing for yourself, no problem cutting corners and making adjustments financially. For potential customers, you can't skimp. No one is going to pay for something that looks homemade.
Machines like those needed to make nice quality products are not cheap (talking the thousands). Designer fabrics aren't cheap, but this is what people are going to want- not mass produced fabrics found in most fabric stores. You're going to have to find the good stuff that will give your product that extra-something. (Google designer fabrics and you'll see the difference between these patterns and fabric choices in contrast to what you'd find in stores.) Computer programs and threads needed for some of these professional quality sewing/embroidery machines aren't cheap either if you go that route. Notions that make your items look professional can be costly too. Sure you can get the cheaper stuff, but believe me, there is a difference in the finished product when you're using the expensive and unique stuff.
If you do have an idea that takes off, have you thought about who your target market is, and how and where to best get this product to them so you can make sales? Would you peddle these on the internet? Out of your garage? At the church bakes sales? Trade shows? Fairs? Conventions? Garage sales? Or schlep to boutiques in your hood that sell this kind of stuff? What's your plan Stan? If you don't have one, you might be dead in the water before you get started.
Not to be a wet blanket, this would be too big of an endeavor for a beginner. If you don't have an immediate need to do this, and you can learn all that's needed to sew well, and you can over the years take your time selling things here and there until you find your groove, then go for it. But IMO this is not something you're going to be able to just dive into and it will be a booming success overnight. You need a business plan. You need skills. You need an original idea that has appeal to the masses or a niche. You need machines. You need access to supplies and people to make this work.