I have DD's 6years and about to turn 4 yrs. Both have been appalling sleepers, especially the youngest. 6yr old is improved now at school.
Both dropped naps at 17 months, neither needed much napping time as babies. My DH and I have tried every single technique out there and for weeks at a time but to no avail.
The children are in bed by 8pm after a bedtime routine (same every evening), no tv before bed, bath/wash, teeth, light down, story/song. They do not actually go to sleep until close to 9pm, sometimes later and sit in bed drawing or reading books or listening to audio books more recently, we are teaching them that if they can't sleep they have to amuse themselves until they can. They are not tired the next day and never nap in the day. They have absolutely no problems at school/preschool.
My almost 4 year old still wakes every night despite trying all sleep techniques. She has a lot of bad dreams which I think is normal for the age.
It is apparent that they just don't need much sleep and we have had to accept this, hard as it is, and find solutions so that we can still function despite the constant sleep deprivation. We make sure that they get a chance to run around or do some kind of activity every day to help wear them out and make them tired.
What you realise as you speak to other parents is that one of the common treads of parenthood is unfortunately sleep problems. You can always spot parents of young children, they generally have big bags under their eyes :)
There are lots of techniques for getting children to bed and sleeping through the night...if you have exhausted these then there is probably not much you can do but know that most children go to bed and sleep through easily once at school.
It is worth having a proper consistant bedtime routine, in my experience it's a good place to start...
hope this helps! :)