I used to have a pom x sheltie who was super intelligent and super loyal, gorgeous dog I'll never forget. I will also never forget that he had ZERO tolerance of small children. I worried about it when DP and I started thinking about children. He was run over just before I fell pregnant, and even though I was devastated about that awesome little dog, and still get so, so sad that he isn't alive today, I am only given comfort by the fact that he would have resented my kids and done something to them, guaranteed. Shelties are very loyal dogs and aren't given to being in families, apparently, so that explains that quite a bit.
Meanwhile, my parents adopted a Viszla from a local person once, but when they brought her home I realised she wasn't pure and she had pit in her. She wasn't super friendly and I convinced my parents to take her back because she wouldn't be suitable around a baby and growing child, or, more pertinently at the time, around their goat.
My sister's previous dog was so, so sweet, but was slow to learn in the obedience club. My BiL and her couldn't get a handle on him (but, realistically, they're not really dog people, even though they have another dog now who is very self-sustaining). My sis had that doubt, because she kept asking me what I would do, she kept asking the trainers until they realised she didn't want to hear what was convenient (only after I'd been there for about 5 days, though!) and played it straight. They said that a pure bred would be more predictable with training and a good environment, but the cross meant it could go either way at no notice.
The dog was sweet, but made me nervous. And I'm a dog-lover from way back! I am also an adherent of Monty Robert's words about dangerous horses and I generalise them to dogs - a dangerous animal is better off dead. It either leads a life of being treated roughly because of it's aggression, or will kill/hurt someone and be put down anyway.
With dogs, there is usually the option to rehome to a non-child situation.
I've had shepherd crosses all my life and there was only one I think wouldn't be allowed to live these days. Yet he was ace with us and protective of us (he bit a neighbour...twice!). My current dog is just awesome and a shepherd cross Dane. Our little dog is a Westy cross - she's become less tolerant of small people (put up with all sorts of things with DS but is very short tempered with DD who doesn't even dish out any dirt to her!), and it's here I keep my eye on because she won't automatically go away when annoyed (she has that "love me" desperation) where the big dog knows he can just walk away and find alone time. I place some measure of trust in him especially know that he has infinite space to run away from an antagonising little boy - we're on a farm now and the dogs aren't yarded unless we're not in!
Like I said to my sister, if it were ME I would move the dog on (I never suggested destruction unless as a last resort - she wanted a resolution before the baby was born in 3 months of that time), but that's ME and ME wouldn't have been fooled by the breed because I've got a much better eye for these things than she does.
I've come across dangerous dogs before in vet clinics and they are very scary beasts. Not pure bred, necessarily, either. The dogs would have been better off dead with the existence they had. I came across them when they were in for desexing. Would be cheaper and happier to give the magic green needle, frankly. The dogs were unstable and had to be restrained to be sedated before being put under general for surgery. They were wearing awful large collars in prescribed colours. These dogs can't actually enjoy life or be enjoyed by anyone. Who knows, maybe they were nice as pups before they turned and their owners too naive or egotistical to put them down.
My GF had her lab put down. She adopted him later in his life and she loved him. Until she saw him grab a chook and completely disregard her commands (they'd been going to training), totally caught up in the kill. She had a small child and a baby at the time. She couldn't live with the risk while he was alive, so she made the awful decision. Knowing what needs to be done doesn't mean it's an easy decision to make. She was a mess. But relieved when it was done.
Early desexing is recommended. Those growth issues can be mitigated with careful feeding - people with large dog pups often make the mistake of feeding their dogs a lot. Careful feeding is the key, regardless - our dog was desexed at 8 weeks and had a shallow hip whilst growing and given the all clear at about 2yo. His siblings were mostly destroyed by 6 months because their dysplasia was so bad - the clinic (where I was volunteering and where I got him) was sure it was because of feeding method. I sought some expert advice on feeding him when we got him and was aware of the potential issues, so he's still alive today Anyway, early desexing is a sound practice and not to be feared.
It's a weigh up of the risk of attack and how prepared you are to take the ultimate, hypothetical responsibility. The guy in St Albans is now going to be charged because the dog was unregistered. He'll be fined, as well.
Good luck.