I'm really looking forward to the "answer" to this as we are having the same problem.
Here is what I "was" doing and then what I am trying.
Tom used to wake at 7am, go to sleep about 1pm and sleep for 2.5 to 3 hours. It didnt matter what time he went down, thats how long he slept. Then he would happily trot off at 7 -7.30. It all turned to custard when Daylight Savings started (but that maybe a coincidence.
Lately he has been looking tired early in the morning, refusing his afternoon sleep, being totally buggered and crashing at 6.15-6,30ish. Its so easy to get him to bed at night so I have been running with it. Its not ideal but hey, it beat having a sleep and then being up at 9pm!!!
However, I have noticed he had been grumpy, argumentative, not interested in eating, not interested in playing by himself, biting his brother and at daycare, wanting to hang out with the under 2s more and more and he doesnt play outside. I am wondering if he is really sleep deprived. His teacher mentioned to me that she thinks he's really tired there. (as a side note, it just occured to me that this could all fit with glue ear which we are checking on Tuesday).
So I decided he HAS to have his afternoon sleep. I have been advised to try a) putting him down earlier...eg 11.30 or b) waking him earlier. Putting him down earlier sounds hard so I am compromising. Today, I picked him up early from daycare (partly because James needed to sleep and I didnt want to wake him etc) so I picked him up at 11.45 instead of 12.45 and took him home, fed him and then with James fast asleep I sat in Tom's bed with him reading stories till he fell asleep...took 5 stories. He fell asleep at 1.30. I am going to pop in around 3pm and see if he is coming out of a sleep cycle then and if so wake him up.
I'll let you know how that works. I figure its a better compromise to let him have 90 min and get up than 3 hours and not sleep at night. I think that was making him more sleep deprived so we were in a really vicious cycle. I hope to break that somehow.
I find (and you probably do too) that its really hard to take the time to get Tom to sleep when James is awake so I have to time things prefectly to get James asleep before I try Tom. At the moment Tom is needing help and I dont really mind that. Its "our" time together reading and I think he needs it. Besides, I guess thats my job right now
I'll be sure to let you know if any of these tricks work.