I think the best favour 'you' can do for your children is to encourage them to save money from an early age. I always told them to 'save some, spend some' with their money from a very early age. ds1 had a job at 15 . He is now nearly 20 and has best part of 15k saved. I know everyone can't Dave this much at such an age but he is years off getting a house yet and won't need us for a deposit.
Adding to what MrsTerryPratchett said about the insecurities of renting in the UK, it's statements like these which show how painfully out of touch some people are with SE property prices.
It's great that your son has managed to save 15k in 5 years, although presumably he doesn't have any dependents (and was living at home for at least some, if not all, of that savings period).
The average deposit needed for a first-time-buyer in London last year was £90k. Your son has saved 3k a year. Let's assume he's got a huge payrise and can now save an average of 1k a month from now on (which is fat chance for most people).
In 2023, he'll be 27 and have enough for a first-time-buyer house deposit -- in 2016. If property prices increase at 10% each year, he'll have about half what's actually required for a 2023 deposit. Keep on saving. Do not have kids. Do not rent your own place. Work a professional job but live like a student, into your thirties and beyond. Keep on chasing prices which were expensive when you started and that are increasing much, much faster than your salary.
The price of property in the SE is so insane that getting a deposit goes way beyond simple, careful saving for most people. It's not a case of staying home and eating cheap cereal any more. You need a jump-up: a windfall, an inheritance, a city bonus or a help from affluent relatives to get that initial huge sum together. By all means teach your kids to 'save some, spend some', but don't let them think this'll make any particular difference if they want to live in their nation's capital, or anywhere near.