Training Classes - CenterMassCombatTactics Firearms Training
b&s personal safety and firearms education
I live in md too. I'm an executive member of MSI and you'll see me around mdshooters from time to time. Md's laws may suck, but we've got a fair amount of good training resources available. The above groups are near you and have a great reputation.
A gun is not magic, you need skills to operate it effectively under stress. When you put on a gun, you are opening yourself to the possibility of being in a gun fight. How comfortable would you feel just buying a sword and jumping head first into a sword fight without any training? What is going to better prepare you to use your gun, $100 worth of ammo or $100 worth of training by a professional? Get schooled.
As for carrying practices...carrying is a lifestyle and you will have to use what works for your lifestyle. The belt, holster, gun, and mag holder that works for me for a given outfit/season/activity may not be what you need. My carry gun changes from an m&p45 w/ tlr1 owb to a m&p9c iwb depending on how I'm dressed and what I'm doing/where I'll be. The main point here is that you need to experiment with gear until you find something reliable and safe that works for you as an individual.
Testing your gear in training and competitions is a good way to refine what you're using. Everyone who carries has a drawer or two full of holsters they used to use until they found (insert current carry holster here) and fell in love. Speaking of competition, swing by the thurmont IDPA shoots and see how you do. The free new shooter orientation class itself may be worth the trip for you.
TCSC - IDPA Matches
As for your original question of if it's safe to carry a m&p with a round in the chamber...yes, as long as you don't press the trigger to the rear somehow when you don't want to. The m&p has a trigger safety that blocks the trigger on the frame if pressed from the middle. It's designed so that the bottom must swing back first(this happens naturally when your finger presses it) before the main body of the trigger and move to the rear.
The m&p also has an internal striker block that prevents the striker from moving forward in the striker channel until the block is moved up and out of the way of the channel. It is pressed up and out of the way of the striker channel when a node on the trigger bar runs in to it when the trigger is pressed to the rear. Again, the gun wont fire unless the trigger is pressed fully to the rear...even if the sear or striker face break.
I personally prevent the trigger from being moved rearward when i don't want it to be by keeping all my handguns in holsters that cover the trigger guard. the m&p45 on my hip right now is in a holster that covers the trigger guard. It won't fire unless it's drawn from the holster and the trigger is pressed. At the same time, if someone breaks in my door right now, I can get shots on the threat(if it requires them) within a second or two.
As for playing with your kids...your kids should have basic firearm safety knowledge by now and know to keep their hands off your gun. Safety hardware is not nearly as effective as safety software. Personally, I'd probably just keep a drawer-safe or lock-box handy down where you often play with them and just toss the gun/holster in there (remember, the holster keeps the trigger from being pressed when you don't want it to be, think of it as a manual safety) when you want to wrestle around with them.
If you prefer to keep your gun on you, get a holster with retention like a safariland ALS or
galco thumb break. When I wrestle around with my dog, or go hiking somewhere where I carry, I like my galco thumb break holsters to both be comfortable and keep my gun on my hip. You'll have to re-train yourself to unsnap before the draw, but it's worth it when extra retention is needed.
Cliffs notes version
1) get training
2) Find gear that works for you
3) Carrying a hot gun isn't a big deal if you keep the trigger guard covered with a holster
4) temporarily store your gun or get a holster with retention for horseplay with your kids
5) safety software is more important than safety hardware