If you want to get into technical terms, I don't think it's possible to be 100% purely positive 100% of the time. Even withholding food from a dog in order to get them to understand that is *not* what you want could be considered aversive.
I am primarily a positive trainer as most on here know. I don't believe any dog should feel forced. There is always a way to teach a dog without pain, fear, intimidation, etc. Is a water bottle going to be a good method for every dog? No. Some may be too soft or too timid. Some may not care. Our JRT mix Lilly found the squirt bottle to be rewarding.
She LOVED it... so much for that.
While I wouldn't say it would be primary method to train a dog from behaving in this way.... I think for a fast result in a situation such as the OP where the mom could be in danger, something like a squirt bottle may be appropriate.