I'm going to be honest with you here, and say I suspect you are at least part of the problem.
Just to set the scene, they now have teens, and actually teens don't want to meet up with family and play at the local park that much. They may do it once in a while, or if they like playing with toddler cousins, quite happily.
But also they take just as much time as toddlers, just you don't need to supervise them as much and they don't go to bed early to give you a break then.
So they may be genuinely busy and enjoying not having toddlers around.
When you have toddlers, you often think lovingly how when they're teens you'll get your life/house/time/lie ins back. And in some ways it's true, but not as much as you think.
I loved the toddler years, but actually I don't want to go back there. I loved playing with toddlers before I had my own, loved my own, but now... relatively unbothered.
Jokes are always made of here we go again, what do you want now
This is what stands out to me. Never done it to their face, but we have a relative for whom this is what we think when they get in contact with us.
That's because from experience they only get in contact with us when they need something or want to show off about something they've got ("we thought you needed to have the broacher from our skiing holiday just in case you needed to get urgently in touch with us" level. No, we have never urgently needed to get in touch with them.).
For example, one year they just had to take us out for our anniversary, no refusals accepted, (don't normally get presents at all from them) and we waited, and sure enough when it came to their anniversary we got a message what they would like-several times the cost of what they had paid for ours. Totally predictable and really irritating.
So think about it. When you do meet up, do you want to talk about you and your problems? Do you expect them to pay for things? You you make broad hints about how your washing machine broke/need a lift to XYZ/something heavy in the car lifted in etc.
Do you spend a lot of the meetup complaining about your things? That's pretty wearing for listeners.
I would suggest meeting my family members usually once a week at the weekend- not every week either
I wonder whether you do it more often than you think you do. The "once a week at the weekend" then "not every week" implies to me it's quite a lot. Be honest. How often do you ask them? Twice a month?
Thing is you may think that you say ask them twice a month and maybe they come once a month. But they may feel that they're being pressurised to meet up and would rather meet happily every couple of months than feel that you think they're not doing enough at once a month.
What I'd suggest is pulling back.
Rather than "would you like to meet up this weekend?" pick one in the future. Say "Would you like to meet up one day over Easter?" then when you meet up, make it your resolution to say 10 positive things for every negative. "I'm tired... but aren't the daffodils lovely." And for every thing you say about yourself, ask them something about them, and listen to the answer.
Give them a bit of space and you may find that they feel more positive about meeting up and want to see you.