Its a tricky one and every child is different in what they need, I had a similar problem at about 16 - 18 months with one of my mindees, wouldn't sleep at all, you could see how exhausted the poor little mite was as well, used to lay on the floor to play as didn't have the energy to move around, mum said wouldn't sleep at night either, would shout and throw things out of the cot, turns out dad was picking him out taking downstairs and watching cartoons, having snacks, who would want to go to bed if this was the alternative, on the other hand I don't agree with trying to enforce sleep, but if the child is obv tired then I think you should try a routine and make sure parents do exactly the same, so that the little one knows exactly what is happening. Some of the things we did to solve the problem are:
A quiet time/wind down time before sleep, read a story (but not an exciting one)
Some soothing music - we put a brahhms lullaby on loop, not too loud just enough to hear
A quiet room, with no distractions, no toys in cot
Calmy say its nap / rest time now (try not to mention sleep, as the child will get worked up before it even gets into the cot)
As you take the child to the sleep area say something like - that was a nice story would you like to paint, play with whatever after we have had our rest time
If the child kicks off, go over, don't make eye contact, gently lay back down and say, it's rest time now, try not to talk apart from this, keep laying back down but don't make eye contact or speak
This method has worked lots for us, the key is to reward the child for resting, sometimes they may just lie quietly and listen to the music without actually going to sleep, if ths is the case, give them 10min or so then go over and say well done, that was a nice rest shall we go and play now, gradually leave it a lttle longer and usually they will have a sleep eventually, you need the child to associate the rest time with something to look forward to or some form of reward so that it doesn't become a battle of wills or something troumatic, but the parents must do exactly the same if the routine is broken your'e back to square one and it's even harder.