Do your DHs ever hit your teenagers
No. Of course he doesn't.
My DH is generally quiet and reasonable but can be prone to argumentative and aggressive behavior, particularly when he's had a drink
Excusing his behaviour because alcohol?
Everything is good in our lives
It most certainly is not.
But recently DH slapped our eldest on the face when he was drunk and she was being what he'd regard as disrespectful
He hit her?
She slapped him back
Good.
In the past I have known my DH to be verbally nasty
Not the quiet and reasonable person you described ^ then?
he has grabbed their wrists firmly in the past
And you did what to protect them from being abused?
I've always told him i wold leave him if he ever hit the kids
Have you left?
I've always told the kids they should never tolerate any abuse from anyone.
But you have let your DH abuse them?
DH is remorseful
Pish.
should I insist on him getting help?
No you should protect your children and leave. If he wants to get help he can seek it out himself.
Had this happened to anyone else and did getting help work?
Most people either leave or stick around for more, which is you?
Should I be worried?
Your DH has physically abused your children and you don't know whether to be worried.
Come on OP, eyes wide open. Be a parent to your children. Protect them.