I think the reason some people have got a bit chippy, OP, is that your posts are a bit frustrating to read. I wonder if there was a reason why you are single and have not had children? It may be that you made a choice to live that way, obviously I don't know if you want to share that information but it might help shed light on things.
I do understand that it must be lonely to spend weekends and free time alone. I think however, your post comes across as though you expect your friends to fill this time for you and are disappointed that they don't. It also seems that your friends are happy for you to join in with their family's activities, which in my opinion is quite friendly and inclusive of them, but that you're dissatisfied because their daily activities don't meet up to your standards of what is fun to do.
Forgive me if I've got things wrong, but that how it came across to me. You said there's no solution, and don't seem to want posters to offer advice of how to make other friendships or find things to fill your time. I understand you may just want to vent, but it does make the post a little frustrating to read.
If you were willing to accept suggestions, I would definitely agree that you should pursue other friendships among people who are interested in doing the same activities as you (which are....? I'm not really sure what it is you would rather be doing with your free time?) I'm sorry you seem to find it hard to make new friendships. I agree that deep and lasting friendships can take a long time to develop, but I don't agree that it's hard to meet new people to enjoy social time with. You say you're busy and doing things all the time - are all of those things solitary things, then? I'm not sure how you can be doing lots of things but not come in contact with other social groups rather than your friends with young children? What about work? Do you not get on with any of the people you work with?
Just speaking personally, I have young children but I like a busy social life, and in the last month I went to the following:
An event at a local pub for people to turn up and draw - a few people knew each other but by no means all.
A Green party meeting with local speakers. There was a social aspect to the meeting.
A low key work drinks thing I organised
A supper club which is for people to meet others locally - it's not like a dating thing, it's for friendship.
I also volunteer at a local arts venue one or two evenings a month, working behind the bar. You said you'd tried voluntary work and found it boring - but there are many different kinds of voluntary work.
I'm sorry you find your friends' lives boring now, but I would definitely encourage you to take positive steps to improve your loneliness and boredom.