Took me a while to work out b&g must stand for one of each gender.
DD has a best friend plus two others ? Sounds a little like myself at that age. Friends I had weren't in my class at school anyway. I think some folk have innate social skills that they build on whilst others, perhaps more bashful and who find social interaction a bit of a strain, learn to accept their own company. I don't believe there is any magic formula but you could assume she is capable of understanding that nothing changes until you make a start at changing them, and that is one wishes to be good at things while one is older, then one has to take a deep breath and get used to things that make one feel uncomfortable now, so they are not still uncomfortable in the future. If the reason for needing to do things is clear it must help encourage the individual to try to tackle the scary things in life. I'm sure she accepts and in a sense is content for things to carry on as they are, but if pushed to think about it might she not admit it would be better to be more socially accepted ? In which case the effort is needed.
DS is not understanding why then. Maybe DD isn't either but doesn't openly question it. But he seems bright enough to realise the characters get more attention, are more fun to be with, and so develop a larger circle of folk who want to be around. He just hasn't worked out it isn't necessarily being naughty, it's being active, full of ideas, and thus fun. If he wishes to be with them, then for the same reason so do others. Maybe he can be helped spot that final deduction.
The issue is to be fun in a play times and intelligent in the learning times. And maybe find something he is interested in and good at to take up as a hobby and be interesting because gives him an aspect not everyone else can boast of.
Intelligence is to do with being able to reason, it can be a help in social situations but it can be a hindrance. Social skills are not the same as intelligence. One ideally needs to have both.
And for sure quality is more important than quantity, but one needs enough not to feel as some sort of oddball or outcast. There's a balance between being an individual and being part of the group.
If he isn't getting a kick at the ball maybe he isn't calling for it enough or being noticeably getting into the game. If he isn't already seen as one of the skilled few then no one tends to think of looking for him. Or maybe football isn't his forte and he should view it as a fun thing rather than something he shines at. It's a team sport, it tends to be most useful for those already popular. Best of luck breaking into it but I think sometimes one has to find a forte of one's own.
I think they ought not worry if they are not the social type. Not much of one myself. But generally speaking one's character doesn't change without effort doing that which is uncomfortable. And also one should find activities that suit them rather than try to be the star at things they didn't get a good start at and are now wondering how to catch up. Maybe something arty. Music, painting, whatever ?
Not sure what else to add really. I'm sure they'll work it out.