My partner is Asian and used to stay with his sister for a few days at a time when he was a young teenager.
his sister is 10 years older then him and has a few children, some are a similar age to DP so he loved staying.
His mother used to send him with bags of food and money for treats, she wouldn’t have dreamt of sending her kids or visiting anyone herself empty handed. Kids are expensive to feed and if someone is requesting the visit then I don’t see how it’s “cold blooded and calculating” to ask for a contribution, the OP isn’t offering she’s doing a favour!
I’d say it’s more calculating to palm your kids off so you can have a break and eat takeaways without offering anything towards feeding them.
My sister constantly asked me and DP to have her kids, she would drop them off and ask for us to have them one night and it would turn into several, sometimes she’d drop the dog off too.
We were struggling for money and the kids were spoilt and constantly asking for days out, takeaways and treats, we were too embarrassed to ask for money but it was costing us a fortune, when we said no to the days out there would be tears and we’d feel guilted into it.
We finally realised we were having the piss taken out of us when we said we were busy and couldn’t have the kids, my sister put them on the phone to beg us so we said we would take them out for the day but couldn’t have them overnight. We didn’t answer our phones when the inevitable calls came requesting that we have them overnight and ignored the texts saying my sister and BIL had an emergency and we needed to have the kids, we just took them home for their dinner after we’d had them all day.
My sister and BIL were not happy when we showed up, they had ordered a huge Chinese takeaway and had a bottle of Prosecco open, there was no emergency and instead we got berated for not feeding the kids!
We went inside with them to say goodbye and my sis and BIL closed the dining room door and carried on eating and refused to say thank you or goodbye.
My Asian DP found that extremely offensive as good hospitality is a huge part of his culture and if you are eating you offer food to guests - especially guests that have been looking after your kids when you simply be arsed with them.
If we hadn’t acted in a “calculating and cold blooded way” by not feeding the kids and dropping them off then we’d have likely continued being used for childcare and spending money we couldn’t afford so that the kid’s parents could enjoy child free time. Luckily we realised at that point we were being completely taken advantage of and turned down any further requests to have the kids, it was a shame but we couldn’t afford the kids expectations anymore anyway.
I think the OP is being very kind and getting completely taken advantage off, It’s the people using her that are calculating, I’m sure most people would show gratitude and offer something to contribute towards feeding their children or offer something to say thank you.