Short answer: it depends.
OP the comments seem more about your "married"/partner vs single status than having children per se.
------
Long answer with some poorly worded examples: Lots of people (seem to) have DH that earn mega bucks. So that might be the pension comment. They assume the husband is subsidising and paying more into a pension than the woman could on her own. This is regardless of kids, and more about single vs married status.
Plus, two people receiving (for example) £15,000 per year from a pension (30k combined) will be better off that one person with a £15k pension income. I.e. even if husband and wife earn the same wage and no subsidising happens. Again more about single vs married, than kids vs no kids.
Similarly, throughout the relationship housing costs, food, holidays etc are relatively less, assuming the partner doesn't earn less (i.e. you're talking to women colleagues earning the same wage, one is single and one has a DH, but the DH earns relatively less - that couple might be "worse off" than the single higher earner due to having to subsidise). Again, single vs married, not kids vs no kids.
Kids are an additional expense, so if everything else is equal: Obviously the couple with kids will be less well off. Perhaps a single person working part time on minimum wage (or on benefits) will be worse of than a single parent working part time on minimum wage (I.e., due to additional benefits for the kid's kids). Or two cohabiting parents working part time and receiving benefits will be better off than one childfree person working full time in the same minimum wage job.
If someone is paying child support they will also be worse off than someone with no kids and child support to pay - assuming both compared individuals have equal income.