I'm a 'just in case' packer OP! Also have a 4 and 1 year old. My rucksack changing bag has snacks, water, dummies, nappies and wipes in one of those folding changing mats, change of clothes for both kids, spare jumper for both, waterproofs, muslins, tissues, hand gel... But that stays in the car permanently. I'm never that far from the car that we can't go back in an emergency.
Buggy Pram Bag Organiser, 11L Large Capacity for Baby Accessories with 2 Deep Cup Holders & Shoulder Strap Used as Carry-On Handbag-Universal Fit All Buggy Models (Grey) [link]amzn.eu/d/308M2IP[/link]
This has absolutely changed my life! I have a pack of wipes, 2 nappies, a baby grow for emergency changes until I can get back to the car, hand gel, dummy, a zip lock of snacks, a bottle of water, my purse, phone and keys. No carrying, no sweating, honestly it is an absolute game changer!!!!