I faced this dilemma too, except my (soon to be) ExDH is away for a lot longer than 6 months, and is in a Cat B prison (but entitled to move to a Cat C, not a violent crime).
I took DSs to visit him. My older DS had been bottling up his emotions, and cried and hugged his Dad when he saw him, and DS2 also hugged and held onto him. They missed him a lot.
Visiting wasn't scary for them, in fact I think it allayed a lot of their fears over where their dad was and if he was okay. To be honest, I've been to hospitals that look more institutional. We moved through in a big group, they were very careful about searching the DSs, and just gave them a light pat down and being friendly with them. (If they've ever been patted down going through security on international flights, they wouldn't find it any different.)
The room had a cafeteria in it, and we bought some hotdogs, burgers
and crisps and had a meal together.
Since then DS1 has been far more open about talking about his dad, and missing him. I think if we hadn't visited he'd still be bottling it up. They are both more open about talking about him with me.
He now calls them about 3 times a week, and chats to them and they're so thrilled to talk to him all the time.