My guess is that she has to take the dog out anyway and can't leave the 4 year old, so is making up some excuse.
How do you know it is 2/3 times every night? I doubt this from your evidence.
That said, if the child seems thin, tired or otherwise not ok, I would alert someone.
This isn't a known cultural thing in the eastern european countries I know (more than one), in fact, most wrap their children up in ridiculous amounts of layer and don't take them out in the cold. They certainly don't deliberately expose them to the cold as they know how dangerous it is.
She could be odd, or even abusive. Leaving a 10 year old at night is NOT ok either- what if he needed someone. I don't think that social services would find a 10 year old left alone at 2am ok at all, or numerous nightly wakings for a small child.
I wouldn't report on what you've seen, as you haven't seen that much. If I genuinely thought there was a real mental health issue going on which there might be as this isn't typical or normal or ok behaviour (to the 10 year old let alone the 4 year old) then reporting might look the better thing to do.