IF you end up sharing a hot tub with a toddler again, point out to the parents that toddlers can easily DROWN in hot tubs if their foot were to get stuck in the suction.
I've yet to see a mom and dad enjoy a meal with a screaming kid, esp a mom. I've also never yet seen in either a really nice restaurant or a cruise, a parent just sit there with a screaming child, the mom usually ends up walking around with the child or removing him or her. I'm seriously surprised if that mom was ignoring the child totally, that the toddler wasn't throwing glasses, plates, cutlery etc around the room. That toddler could have started a unique game of "darts" with steak knives. What you've seen is an exception of kids and parents in the dining room, not the rule. Fortunately cruises do have other dining options if needed. That's one of the reasons cruising is so great for families, there ARE choices.
Most parents are used to adjusting their lives around their children and that doesn't change, at home or at sea.