As parent of four, now ranging from 10 - 19, I'd concur with most of the sentiment here. After 8pm, its unfair to expect a very young child to be 100% predictable - most will be getting cranky, or at least unco-operative - it goes with the job description! - so any parent that expects to inflict the gamble of a tired and emotional toddler on fellow adult diners is chancing their arm IMHO. A very quiet (as in almost empty) venue may well have a quiet, appropriate corner to spare - so that an disturbance can be minimized, but it remains the restaurants choice whether they can monitor the situation to the mutual respect of ALL diners.
Even daytime dining requires social respect for other diners, unless its specifically a family-orientated venue that encourages a degree of hullabaloo? A restaurant isn't a library, but its not home turf either. Large groups can become very selfish, I think. If a bunch of parents are sat quaffing, while the dining room becomes their kids' creche, I'd consider it acceptable for the manager to ask if one of the parents could bring the mini thugs out for a bit of air while they finish up? I appreciate this is a hospitality minefield though.
I'm fully aware that some children (puffs with pride) have a good record. And I'd guess experienced staff can pick up on potential angels and devils pretty early on. But evening time must be effectively policed - in advance, if necessary - flagged on websites and entrances for sure - and always politely and firmly?