Missy makes a good point, if you have not had basic first aid training including cpr and Heimlich, you need to do that before you start keeping other people's children because beyond the legal ramifications, you don't ever want to have something happen that you would spend the rest of your life living with. My daughter wound up having a breathing incident that put her in the hospital at 9 days old and they taught us the infant cpr before we went home but I wondered, why when they won't let you out of the hospital without a car seat, do they let you take the kid home without the basic life-saving knowledge of cpr? Your son is small, you also need to go through your entire house and make sure it is child-proofed adequately for older children, which means getting down on your hands and knees and seeing all the possible dangers that you haven't dealt with because your son is not yet ambulatory.
If you earn more than around $300 a year doing anything you pay taxes on it. Watching 2 children that are not your own does not require being licensed as a day care because it is considered babysitting. I assume that your husband's colleagues are going to claim the day care expense and unless they pay you in cash there is a paper trail.
I have been working at home for 13 years, keeping track of the receipts is easy, throw them in a folder and get your paperwork together when you do your taxes. Ihave never paid quarterly taxes, it is a choice. You can pay them at the end of the year and essentially, what your husband pays in that would be refunded is applied to your social security. One thing I would recommend is setting something aside every month in case his overage doesn't cover what you should pay in. That way if you have to write a check, the money is there. Also, don't ever claim anything you do not have a receipt for.
I have numerous friends, roofers, other jobs, that are considered self-employed and they will claim stuff they have no receipts for, which is fine if you never get audited but not only will they up your net income to match the undocumented expenses, they may decide to crawl up you know where with a microscope. Much easier to bite the deduction if you can't find a receipt than to chance it.
Also, you will be able to take a portion of your mortgage and all utilities to the business without ever registering as a business or licensed daycare if there is a portion of your house that is pretty much used for nothing but business. Add up the square footage of a play area, if you start to keep filing anywhere. If you get a shelving unit for the garage to keep the children's belongings. Add the square footage of that, divide it by the whole square footage of the house, and you have your percentage of utilities and rent that can be billed back to the business.
You might be better off asking the guys to pay with checks because you can easily copy them on your printer (if you don't have one that is a printer, copier, scanner, you can get one for under $80 and it would be a good tool for you). That allows you to keep easy records of the income that you should create a microsoft word doc to give to your CPA or H & R Block. One for income, and several for the expenses. The rent/utilities, receipts for food, wipes, kleenex. That means when you pick up something at Walmart, circle the item and throw it in the file.
Hope this helps and feel free to contact me if you have any questions. This doesn't have to be complicated but if you don't keep good records, you are not taking the appropriate step of reducing the income by the expenses to run the business. Also THINK OF IT AS A BUSINESS NOW. It took me several years to really get that I owned and ran a business rather than some little making extra money gig. That thinking will be helpful in making sure everything is cool with taxes, etc. Also, as I have had friends do this, you need to set up rules like a regular daycare.
For example, when they will be dropped off, if they plan to keep a car seat with you or leave it when they drop the child off, or do they have a problem with you running errands, goin to the park, etc. with the children. What time will they be dropped off and picked up. I would specify that an occasional problem that will leave the child there late is not a problem but if it becomes chronic, the easiest way to stop it is to charge them for picking up the child late. Think of every hitch that could happen. If someone forgets formula, etc. You need to figure out how far your responsibility goes for taking care of the kids. Don't make the mistake of assuming that people won't take advantage of you because they are friends. I am not saying they will do it on purpose, but friends who have done this have had problems setting boundaries when they didn't really have them to begin with and then had to implement them because the clients start acting squirrely.
One of my friend's clients was a hair dresser. She was chronically late dropping the kids off and often picking them up if she had a walk-in when she was supposed to leave. It got to the point that she finally refused to sit for her anymore. Also condsider what you are charging. While three kids is doable, you need to remember that part of what you are getting paid for is the attention you are no longer going to devote solely to your son.