having an 11&9 year old and then a baby will definitely divide up the family when you are together because the baby cannot do what they can do
OP, you could be me really except I'm a bit further down the line. We have our DC now, my DSC were exactly the same ages as yours (both boys too) when our LO was born.
I have to say I actually disagree with the above comment. I found the age gap between them to be precisely the thing that prevented the jealousy and upset that your DH is concerned about.
The fact the baby can't do what they can do yet is good (for us anyway). They aren't jealous of Dad spending his non contact days at a soft play or baby group! And baby isn't jealous of Dad spending time playing Xbox games with his older two or football in the park or whatever. The age gap between them actually means they get to spend time with their Dad doing things they want to do without a young sibling always trying to join in or always being forced to do things more suited for a younger child and vice versa for our LO. And I don't mind being 'left holding the baby' for a little while on his contact days so they can do something together. We spend lots of time together all of us too, it doesn't bother me if he takes a few hours out to do something alone with them as well.
It also meant that they were old enough to be a lot more involved in caring for their little brother. The eldest in particular loves it. When he was first born would sit on the sofa for hours watching TV with baby on his shoulder rocking him to sleep. It was really sweet.
I think we'd have had a lot more jealousy issues had the children been more similar in age i.e. a baby and toddlers.
In terms of practical things we did... we didn't actually do that much. We found it a better approach not to make a huge fuss about it. They weren't included in choosing the name, they didn't decorate the room or anything like that. They asked questions and we answered them when they did. They came to meet him after I got home from the hospital but didn't actually sleep over for the first few days (and contrary to MN belief, they weren't scarred from this either). Even from a more practical point of view in that it was the beginning of the week and they had school so we all agreed it best they not be kept up by a screaming baby whilst we found our feet for the first few days.
One thing though that I did insist on was that I wanted DH to tell them about the baby on his own. I get on really well with them but I wanted them to be free to ask questions and be upset if they wanted to be and not feel bad about it because I was sat there. As it turned out as soon as he brought them home from telling them they ran upstairs to me and excitedly asked to see the scan pictures!
Honestly I don't think we give children enough credit sometimes for their understanding. I can certainly see how these situations can go wrong but also, I think a lot of the time the worry can be for nothing.
In our case, everyone gets on really well, the kids see me and their mum chatting and being friendly regularly. When baby was born, they saw her congratulating us and coming to meet the baby herself and so on. They know their Mum is happy and we get on which I think makes all the difference (and is what has always allowed them to have a great relationship with me too).
There was just no reason for them to feel pushed out. Life has changed for us yes, but our treatment of them has not and I'm confident that the love and care they feel when with us has not either.
Your husband is right to think about this stuff. My husband did. But honestly, I wouldn't think too much about it. I actually think you run a higher risk if you make a massive deal out of it.