"But it would help if dog owners could occasionally recognise that shouting 'he won't hurt you' as their dog bounds disobediently over 100m of park to lick your child is, dare I say it, inflammatory to the parent who is trying to allay general terror of dogs in their child."
Difficult one, this. I agree with you, but... I walk my dog in an area that includes a field (nominally a sports field, but the most I've ever seen is a couple of teenage boys having a kick about) and a small, enclosed play area (swings etc). 330 days of the year, there will be no one but dog owners in that field - and only once or twice a year will I see children in the play area. So I and other owners have their dogs off the lead coming up to the field. No one is realistically going to keep their dog on a lead on the remote off-chance that there are children about.
On one occasion, my dog was happily ignoring a small girl, until she started running in the other direction - which obviously triggered his predatory senses and he ran after her. He did only want to play, but I appreciate she didn't know that. I would have caught him and put him on the lead if I'd been fast enough. So then you have a dilemma: if you run after your dog, he thinks it's a game and won't come back to you and will carry on cavorting round said child; if you walk off and wait for him to follow you (which he will), the parents are going to go apoplectic.
Yes, I appreciate that a dog should be better trained than that - but terriers are bred for independent hunting and aren't terribly biddable.
What bothers me, even as a dog-friendly person, is these great slavering Rottweilers or Alsatians (or other dogs of a similar size) pulling hysterically on a lead, when the man holding them can hardly keep them back. With a dog that size, you have to rely on them knowing that they're subordinate - and I'm not sure that some I've seen do.
Aside to Alicemama: Not wishing to intrude, but has your puppy has his second lot of jabs? I've always understood that they shouldn't be out until then (i.e. three months) - but your breeder may have done things slightly differently.