Sometimes, they just need to believe that the sacrifice is worth it, or that they are actually capable of achieving the goal. Remember, that they need to realise that even small savings, on a regular basis, can equal a large amount if left to grow.
I have 2 kids. Both started part-time work at a very young age (8 & 10) in our business. I told them that I would let them work, but they had to save half of their earnings. This money is to be put aside to purchase their first house.
They had no problem at first, being so young and having spending money. However, later when we moved & they had to get "real" jobs from non-family they both struggled big time. Part of the problem was lack of shifts, so at 14 years of age, they had less money than at 10. Another problem is that so many kids get Youth Allowance for doing nothing, so they had less money than than their friends. One of them used to resent me taking half of her money and would have regular arguments with me about it. Neither of them were allowed access to the money.
It wasn't until each of them were around 17 and their respective savings were quite high that they really started to appreciate it. There is nothing like the knowledge that this nest egg is growing constantly and that they have more in the bank than many adults twice their age. The eldest is now 20 (this month) and has a large deposit set aside. She is not earning a huge amount as she is doing a traineeship. If she was in a normal job, I believe she would have already bought her first property.
The youngest, just turned 18 and is doing her HSC. She has a deposit for her first home and is quite excited that she might be able to purchase it next year. Of course, this depends on what job and income she gets.