Q. My three children — all licensed drivers — are home for the summer and must share the same car since my husband and I can only let them use our cars occasionally.

We do have a family car however and are trying to let them work out a plan so they can have equal use of it, but how do they pay for the gas when they only have part-time jobs?

They need some ideas — and so do we.

A. The sharing of a family car can be tricky because your children aren’t children any more but they’re not really grownups either.

Instead they are in transition which means that you and your husband are in transition too. Your basic parenting work is done, but now you are in the “polishing years” when your children need you to help them smooth away some of their rough edges.

They may know how to study and how to organize their time at school but many of them still don’t manage their money too well; they don’t think about consequences as much as they should and they are usually a bit more self-focused than they need to be, especially when they first get home.

If your children are like most young people, they regressed the minute they walked through the door and they didn’t even know it. When they look in the mirror, they simply see three young people who make their own decisions at school, who know more than their parents know and who think that all requests should be made to them with delicacy and tact, which isn’t a bad idea. People are much more likely to do what you want if you treat them nicely.

When your children go out at night, ask them to give you a kiss when they come home so you won’t wake up and worry about them, instead of telling them that you want a peck on the cheek to find out if they’ve been drinking or smoking tobacco or pot.

There’s no point in setting a curfew for them however since a few grisly mornings will teach them to come home early if they have to be at work at 6 a.m.

And when they have a specific problem — like scheduling the use of the family car — ask them how they’re going to solve it, rather than telling them what to do, but don’t expect perfection. There is sure to be some bargaining, some raised voices, some “he said; she said” while they work out their differences.

This will have its own reward. If your children learn how to negotiate, to compromise, to cooperate — and to apologize when they’re wrong — they will know how to negotiate, to compromise, to cooperate and to apologize when they work in an office or play on a team or work through problems in a marriage.

You should set your own rules about the gas however. Tell your children that the family car is like a rental, so they should return it with a full tank of gas each time they use it or pay twice as much per gallon for someone else to fill it up.  When they borrow your car or your husband’s car, however, they should just replace the gas they’ve used since you won’t want to top off your tank every day because one of them might want to use your car.

And if your children don’t make enough money to pay for the gas? They can walk, ride their bikes, take the bus or if they need a car bad enough, they can look for a second part-time job or they can ask you to hire them to do some extra work for you.

Perhaps one of your children could do the grocery shopping and another could spend an hour or two a day cleaning up the yard or washing some windows.  Your children could also drop fliers in your neighbors’ mailboxes, offering to walk their dog and water their plants, but they’ll get better gigs — and make more money — if they can update Quicken accounts, scan photographs into a computer or find the best apps for a smartphone.

It may sound heartless, but the more you teach your children to cut back on their expenses or work harder to pay for them, the sooner they will learn to live within their means.

To learn still more about growing up and growing older, you might also leave a copy of “Getting from College to Career” (Harper’s; $14) on the dining room table. It’s written by Lindsey Pollak and it’s quite good.

Questions? Send them to advice@margueritekelly.com.