Do send him a list of basics that he will have to cover for the kids, like -
Food x 3 meals a day plus snacks, drinks, give examples and remind not junk-food fast-food options. Fresh fruit, veg, fish fingers, peas, mash, blah blah EVERY DAY
toiletries and suitable bath / shower / teeth products, hair brushes, fresh pyjamas, blah blah TWICE A DAY EVERY DAY
Clean clothes, extra clothes, underwear, footwear, and the laundry taken to keep those things clean and fresh
Bedding, towels, and that they need to be fresh and changed every week.
Facilitating any weekend arrangements any dates the kids have (birthday parties etc) including providing gifts, cards, and generally showing up.
Completing homework, being helpful, RAISING THEIR OWN KIDS, etc.
Contact details for emergencies like their doctor, dentist, plus having basic first-aid things in the house like plasters, antiseptic, and pointing towards what-to-do-when-kid-falls-off-a-swing.
Notes of any allergies, food dislikes, blah blah...
Obviously all the above is just the tip of the iceberg that he has no idea actually exists rather like the captain of the Titanic.