DD is 7. She got some money for her birthday (2 x £20) and then said she wanted to save it for when she is big so she can buy a house. 
We are not short of money but are careful with it, don't buy the kids stuff unless they really need it (clothes) or finish a star chart (small treats).
Tried to explain that houses really do cost a lot of money and that she can spend her birthday money now, and save for a house once she is grown up and gets a job.
Now she is asking which jobs get the most money so that she can buy a big house. And now DS (age 4) has engaged with this too, both seem too obsessed. Currently DD has settled on being a doctor which is fine, but now is asking what earns more than a doctor.
What have I done wrong?