I agree with most GN-ers that 5.00pm is not a natural bedtime, although Manxgirl also mentions 6.30pm-7.00pm which seems seems more suitable. The little chap would then likely sleep on for an hour or two in the morning and the parents could go to bed more in sync with the children.
My DD2 took to - in my old eyes - pampering her awake-in-the-night son. If he awoke at say, 3.00am, he would be offered a banana and stroked back to sleep. This then became the norm until he was almost four and a bit.
I love my children but we did stick to the bath, book, bed routine, at more or less the same time (7.00pm) and they slept through. Unless we thought they were ill, we ignored any mid-night murmurs.
Children will 'try it on' which may be because a child feel insecure but more likely, it is a battle of wills and having a parent mid-night is the ultimate prize.
I actually speak from experience. I am ashamed now to admit that as a more or less permanently sick child, I can remember making my parents - I usually wanted my father - come in to see me when I was awake during the night. When, snuggled up on my father's chest and he fell asleep, I would wake him up again! Horrible brat that I was! If Daddy, you are sitting on a cloud, reading this, I am sorry!
The clocks mentioned sound wonderful, I was not aware of them or would have bought one to help my poor beleaguered daughter and husband. Now their son is fine and sleeps through so there is light at the end of the tunnel.