That’s just it though, three women used to kid wrangling without paid help would have spread the load better automatically without even discussing it. The mother of the babe in arms would take them, and their handbag. The toddler-tamer would likely take the toddler and the toddler bag and their hand bag, and the remaining person would pick up ALL the other bags and we would ALL say thank you to the staff on the way out, including the toddler (who would be encouraged to carry their own bag!)
I think what probably happened was that the toddler was walking, but needed/wanted to be carried the last bit on the steps. Again, if I’d been out with my cousins, firstly we would have anticipated the toddler to need help on the steps and still distributed the bags to the childless person, but if for some reason that hadn’t happened in the first place, The toddler wrangler would have picked up the toddler, and put down the bags, knowing there was someone with free hands who could sweep them up. It’s likely that the child-free person would notice the toddler stalling/fussing and offer to carry them or the bags right off. But Boris didn’t notice, didn’t offer and Nanny didn’t expect him to, because it’s just not his responsibility.
In my experience women look at a situation, anticipate EVERYONE’S needs in the foreseeable and distribute the load accordingly, usually wordlessly. It’s a learned skill. There is no reason men can’t learn it, they just choose not to. Often because the consequences aren’t their problem.