Step parent so will prob get flamed but here goes.
DSD1 who is 13 told me today at 15:50 (Sunday) that she has cooking tomoz at school and needs ingredients.
They were also getting ready for the 2.5 hour drive home. OH texted their Mum and asked if she could grab what she needs (I suggested that as we had been in town she could of mentioned it then - OH proceeds to tell me she asked him yday) her Mum said no it is our responsibility as our weekend.
Anyway I quickly took her to Tesco express and got the things she needed that we didn’t have so she didn’t get into trouble.
I have also sorted it out for her in the past.
AIBU to think the RP should really be sorting this out. It never once crossed my mind that my ex would provide ingredients for school cooking as he pays me maintenance (as does DH to his ex) also the practicality of transporting says ingredients Home.