OP if they decline no need to suggest something else, though it is sweet of you. Leave it up to them to offer to do something else to celebrate. Otherwise, who knows what they have going on weekends and you don't them to feel guilty for declining and like they must make it up to you. Not your intention at all I know. You sound incredibly thoughtful actually. I just wouldn't assume you are doing them a favor by offering to celebrate another time.
I don't want them to feel like they have to do it to make something up to us, but the kids will get together eventually on a weekend anyway. I definitely don't feel like I'm doing them a favor because DS will want to do something to celebrate with this friend, one way or another. I wouldn't say they were best friends, but they're pretty good friends.