Basically, the term implies that people who do have kids are not free and are somehow encumbered.
Well they aren't free, that's patently the case. They are 'encumbered' by their children.
If they want to do something they have to factor in the impact on their children. And I'm not talking about someone jetting off to Rome for the weekend, but just basic practicalities; if you want to go to the supermarket you have to work out if you're going to take DC with you or leave them with someone else. A trip to the supermarket with children is markedly different to one without.
If you are at work and nursery call you because your dc is sick - you have to leave or plan who else will get them etc. Children can and do negatively impact women's careers/job prospects (But that's separate discussion).
If you are getting ready for work in the morning, if you're childfree you pretty much just get ready and go (faffing aside); if you have dc you have to negotiate them getting ready and fed and also getting to school/nursery on time.
If you're cooking - you have to consider what they will eat. When you're childfree perhaps a shot of whisky and a pot noodle will suffice.
When you have children you relinquish a huge amount of 'freedom'. That's exactly what happens.
And get away with this "most vulnerable members of society" trite. You are back-tracking, and are conflating and confused.