This is a topic I'd bring up casually. I suggest you be on casual friendly terms with them so your offer is a part of a casual conversation.
Everyone on my block knows each other. We wave hello to each other when coming and going. I have seen papers pile up and have either put them inside the screen door. If I didn't know them I would leave a note saying I was concerned about burglars and have their papers and mail. But I'd only do this if I had at least a passing relationship with them.
I suggest you find ways of making their aqaintance. When I was new to the neighborhood neighbors made a point of welcoming me. One neighbor brought me a desert. We chatted on the street. For quite some time now we watch out for each other. We have each other's phone numbers. I think that got started when I had a Block Party put on by the police departments Crime Prevention Department. You might consider seeing if your police department does this.
Another idea. If you do not know these neighbors casually enough to be comfortable talking with them. You could have a belated welcome to the neighborhood and include your observation about the papers and suggest you could take them in. Or use this opportunity to get to know them. Next time you see them out greet them and say you'd like to get to know them. Tell them a bit about your family. If they seem friendly then mention the papers.
Because all of my neighbors and I have built up casual friendships we tell at least one neighbor when we're out of town. A couple of summers ago a neighbor noticed an open window. He called the police who then called me because he had my phone number. Since then I've given him a key. This event helped us get started on a good friendship.
A neighbor has called me twice when an ambulance was in front of my house. Once when I wasn't home. We tell each other about suspicious happenings. We look out for each other in a non-intrusive way. Mostly info is exchanged on the street as we come and go.
We've had block parties in the summer. The city puts up baricades and we have a potluck dinner. Those have help build a cohesive neighborhood in which I feel safe.
It's good you notice what is happening. Every neighborhood needs at least one person like you. The more eyes the better in preventing crime.