I have similar aged children and I suck at cooking but am gradually getting better with practice. There have been some horrible dinners along the way, but I can now get ok food on the table without having a wobbler and I'm really pleased about it. Bear with it op, because it is a skill and does take a bit of practice.
Initially I found things I could mostly do ahead of time then warm up were better, so that I wasn't getting flustered when the kids were hungry (stuff like bolognese or tomato sauce for pasta/ to top pitta pizzas).
Some slow cooker recipes are shit, so don't take that personally. Some are great though, and really good for the novice/ shit cook. I like the slow cooker lamb recipe in Nadia Sawalha's Disaster Chef book. If you know someone who uses a slow cooker a lot maybe ask them for their favourite family friendly recipes, or see what slow cooker books they have at the library.
I've found it helpful to get the hang of omelettes (I learned from Delia's How to Cook Book 1) and boiled eggs (dippy for lunch at home, hard boiled for picnics/ packed lunches). Basic, I know, but even doing dippy eggs has had me swearing in the past.
Maybe try nuts instead of crisps with lunches? My kids like unsalted roasted pistachios or almonds (to be clear, they prefer crisps).
Good luck! Mistakes along the way are to be expected, and you'll be really pleased when a meal goes to plan.