We don't treat children well because of their intelligence - otherwise, no one would care about newborns. We care about them because they are of the same species as us.
As for "dignity", I am not 100% sure what you mean, but in humans it comes directly with being able to function as a member of the society. For example, someone who cannot navigate city (be it because they are young or because of their developmental difficulties) will not be free to wander wherever they want.
I think of “intelligence” in a similar vein. We can recognize it when it’s close enough in proximity to the path we humans have taken. The further away another species is from that, the less we are able to recognize/judge their level of intelligence. I like this metaphor because it emphasizes the limits of our own abilities to understand vey foreign things. That’s not to say that we could never get better at it, but there will almost certainly always be more outside of our circle of understanding than inside it.
I’m also reminded of the drunk searching for his house key beneath a street light. It’s not where he dropped the key, but he’s searching there because “that’s where the light is.”
But I think some things are always common - such as not dying from hunger, predators or bad weather. If something as simple as bad weather can cause species to die, _and_ we know there a really simple workaround (houses), and yet there is no sign of it and species just keep dying, can we really call them "intelligent"?
If we re-define "intelligence" as "smarter than a human baby", then this word covers most animals (and probably some plants too) and becomes useless. When doing species comparison, I think it only makes sense to compare adults to each other, and perhaps even "average" or "75th percentile" adults.