Ah, the old situation: you've had a major, perspective-altering life change, and people on the other side of the invisible bubble simply don't get it. Are they simply unable to get it, would they get it if explained, or are they wilfully ignorant? A bit of each, depending on the individual.

I'm not a parent but I don't find it hard to empathise. With some imagination, and reading other's experiences, I can see clearly that being a parent is a huge responsibility, and many tasks you could once do on a whim are now major inconveniences.

However, most people are not especially empathetic. They are overconfident. They jump to conclusions. They're not aware of their own ignorance. They think that if they were in your situation, they could easily solve all your problems. It's annoying. It's especially common amongst people in their 20s, who've achieved some success and haven't been hit with any major setbacks or responsibilities.

(I am 28 but had a major series of blows in my mid-20s that gave me a lot of perspective. Many people my age now seem ridiculously sheltered, ignorant and arrogant. I'm surprised your coworkers seem to have kept that arrogance into their 30s, but maybe it's because they're childless).

Anyway, there's no snappy one-liner you can give that will make them realise this. If you can patiently explain and give some context, some people will get it. (Some won't, no matter how much you explain. They'll keep arguing with you and trying to convince you that they could run your life much better than you can). The best short answer, as noted in another answer, is to make clear that it's your preference:

No, it's another weekend playing with the kids. To be honest, once I became a parent, going out partying just wasn't as important to me.

Whereas if you say "I have to stay home with the kids", it sounds like it's a problem that's been forced on you, and people will trot out their solutions.

There's another subtext in your question that I want to comment on, however. You sounds resentful. Being a parent makes it harder to go out to parties. It shouldn't make it impossible to have hobbies or read books for pleasure. Saying that those things are "childless couple things" suggests that you think being a parent is incompatible with having any life of your own. If possible, you should look into fixing things so you have some "me time" built into your daily schedule.

Basically, how can I tell non-parents they just don't understand, and when they are a parent they will get it.

This might just be awkward phrasing, but it sounds like you are taking a confrontational attitude. There's this thing called "non-violent communication", and one of the principles is avoiding making negative assumptions. Your coworkers are making negative assumptions about you - e.g., that you could go out if you were really organised. In turn, you are making negative assumptions about them - "they just don't understand". This is a recipe for hostility. You'll have a more pleasant time if, instead of asking

how can I tell non-parents they just don't understand

you ask