It may be hard to find a dobe from a reputable breeder who will send a pup to someone without a fenced yard. If you can establish a relationship, or show a past history of active dog involvement to a breeder so that they trust you will keep up your end of the bargain to keep your dog properly exercised, you *might* be able to find a breeder, especially if you can show them a plan for how you will meet the dog's exercise needs. But if a breeder is willing to sell you a dog with no questions asked about fencing and housing, your lifestyle, the kids ages and other pets, they're not reputable.
Which goes to the following you need to know--dobes require a LOT of exercise. About an hour every day, preferably off-leash, is generally about the least you should figure on. They are not good dog park dogs, so that is out of the question. Jogging, biking, etc are options (though dobes are very heat sensitive, so you may need to find another outlet in warm weather), but not
for a dog who is younger than 18 months, so you'd need to find another way to exercise your puppy.
Obedience training and scent work are good mind exercisers which can help keep the dog occupied, and go some ways toward getting him the physical exercise and the mental stimulation he needs, but it is not a substitute for physical exercise. With children and no experience in dobermans or IPO training, I wouldn't consider that to be a good option. IPO training takes a lot of time and strong commitment
(and a quality training club); it might be tough for you to even get enough time with kids in the mix, let alone the rest.
I had a very similar situation as VZ for my first dobe in terms of places he could exercise, though I didn't put in as much time--but it was at least an hour a day Every.Single.Day--no allowances for bad weather or lack of energy (from me). We had no kids; he was our baby. In addition to the daily off-leash exercise in open fields around our apartment, he went with us on hikes (off-leash--it was a long time ago when that was more of an option than now) every weekend--and obedience training. He was happy and well-balanced. I don't have a fenced yard now, and actually that is one reason I got out of dobes. It got too hard to find a good place to exercise them. We managed, with a lot of work, but I'm getting older
and wasn't sure I could commit to exercising the dog so much.
If you are looking for guard duty as a requirement, dobes aren't the best breed for that. They are personal protection dogs; dobes with good temperaments will protect their people naturally. In order to do that well, their breeding leads them to DEMAND personal attention; they are not the kind of dog you can just throw into the backyard to patrol your property. They are indoor attention-seeking dogs.
I personally think having 3 kids with the youngest at 7 is a good group of ages to start with. Kids can really get a lot out of working with dogs, training them, playing with them, etc, if they are interested, and the dog generally will benefit too. The only caveat to consider there is whether you are already so busy with their schedules (athletics, school etc) that you can't pay as much attention to the dog as he needs. And of course, you also need to know and accept that you can't depend on the kids to do everything to care for him (exercise, pick up poop, even feed regularly), even if they promise cross-their-hearts-and-hope-to-die that they will. LOL You'll need to be quite involved.