Clean bedding, clean towels, tea towels, heated towel rail
Washing supplies and cleaning stuff - e.g. Dettol, bleach, dishbrush
Lovely toiletries in a little basket in the bathroom and spares of anything essential e.g. tampons, wipes, loo roll
Well stocked first aid kit
BBQ, paddling pool, garden games/toys - get giant Jenga/chess?
Bug repellant
Hair dryer, microwave, decent freezer, toaster, good kettle
Instructions for every appliance
Board games, books (go for well known authors, e.g. Marian Keyes, Sophie kinsella, and cheap holiday reads), comics and books for kids, some basic toys - Lego, dolls, etc. Sometimes something 'weird' might work well - so for example, something older/vintage-ish that young children won't necessarily have played with before.
TV with netflix or sky demand or something
Store cupboard/pantry with stuff that might be useful for food - e.g. herbs, spices, bottled water, sauces, vinegar
Easy clean sofas and carpets and rugs
Decor in the style of whatever the cottage is near - so if the beach, nautical with fish in the bathroom. Children's bedroom(s) in child friendly decor - maybe little mermaid, or pirates or something if near a beach..
If near a beach - seaside stuff, wind breaker, pop up tent, if near woods or hiking then maybe binoculars or something
Outdoor storage for bikes and the like
Wooden or tiled floors in main areas for easy clean
A Hoover
Local info - takeaway leaflets, tourist info, contact details for owner, emergency numbers (e.g. doctor, casualty) etc