Q. Our 27-year-old son works in the daytime and his girlfriend, who’s 19, works at night, so my husband and I care for their 16-month-old daughter at least three times a week.

We don’t mind watching the child because she is such a joy to us, but our son is being increasingly difficult. Maybe he just wants to annoy us, but he has been the poster boy for oppositional defiant behavior ever since we adopted him in 1991.

We actually cringe when he comes into the room because his confrontations are close to explosive. This is especially true at night when he picks up his daughter. He seldom arrives before 9 or 9:30, even though he gets home in the early evening, and this forces me to call and ask him when he is coming to get her. Sometimes he tells me that he’s late because he had stuff to do at home or that he had to fix his car, and sometimes he lies and says that he had to work late. When he does arrive, he immediately fusses at the baby because she’s whiny by then, but that’s only because she’s worn out.

My son also gives me many orders. When I tell him that his daughter should have been in bed hours ago, he either ignores me or says, “Don’t tell me what to do.” When I won’t let her play near the fireplace, he says, “A few scrapes and bruises never hurt anyone.” And when I hold her, he says, “Don’t do that. She’s not a baby anymore.” My husband finally told him, “Don’t tell your mother what to do. Don’t tell her she can’t hold the baby,” to which our son replied, “She’s my daughter!”

Everyone reacts to his wrath in different ways. My husband thinks we shouldn’t watch the baby anymore, which would devastate us. His girlfriend rants about my son on Facebook and wants to break up with him, but he won’t leave. His baby ignores him as if he wasn’t even there (but shrieks with joy when her mother walks into the room). And my stomach gets tied into knots when he’s around.

(Hadley Hooper for The Washington Post)

What can we do?

A. Maybe you and your husband could walk away from your son, but please don’t walk away from your granddaughter. She needs you — and the needs of a child come first.

Your granddaughter may be able to ignore her father’s verbal abuse now, but your support will be vital when she’s between 18 and 36 months old and again when she’s in her early teens: the challenging years.

She will probably never know why her father is so difficult, however, and neither will you. Perhaps he had a bad start in life, or his chemistry is out of whack, or the fears and responsibility of fatherhood are scaring him witless. Whatever the cause, neither you nor his girlfriend is likely to change him.

You can change the situation if you’re willing to work with the girlfriend, even though she is young and inexperienced, rather than work with your son, because your granddaughter thinks that her mother is a good mom — and she’s probably right. Even if she’s not, you should help this young woman as much as you can. If you don’t, she may not let you see much of your granddaughter if and when she and your son split.

Begin by asking her to drop by for lunch so you and your husband can have an earnest, honest, heart-to-heart talk with her while the baby climbs from one lap to another. She needs to know that you’ve read her Facebook page and that it sounds like your son is making her life even harder than he’s making your life. But if you change your tactics, the situation may improve.

Could you or your husband deliver the baby to your son — fed, bathed and in her pajamas — instead of waiting for him to pick her up? This could make the evening much calmer. Or could the baby stay overnight with you after spending the day at your house? She could get more sleep, her mother could pick her up in the morning and your stomach would be knot-free. Or could the baby stay at your house for one or two weekends a month and be in day care during the week? This might be the best option, if you can afford to pay for all or part of its cost, and it might teach your son a lesson if he is the one who has to pay for every minute that he is late to pick up his daughter. Parents soon learn to watch their watches once they pay for their delays.

If none of this works and the problem gets worse, either you or the girlfriend will have to bring in the professionals. This might range from a parenting class to a psychotherapist to Parents Anonymous — that excellent group dedicated to preventing child abuse and neglect — to the police or even to the courts to get a restraining order, forcing your son to keep away from his own child. Abuse can neither be tolerated nor excused.

Send questions about parenting to advice@margueritekelly.com.