I buy kids size 5 for me often. It's not true they are narrower or made of cheaper materials, they are cheaper because there is no tax on children's clothes. Kids feet at size 5 are no narrower than adults. Where you will really save money is socks! Years ago I bought some great ankle socks for myself from H&M for about £8 a pack of 4, and then the next day realised the Lidl grey school ankle socks in size 3-5 were about £1.50 for 10 pairs and basically identical to the ladies H&M ones. I've never bought adult socks since. Kids trainer socks in Primark are mega cheap too.
My current favourite trainers are a boys pair from Next earlier this year, they cost me about £4 in a sale, they are brown suede with elastic laces (that look like real laces) and the sole is lightweight, sort of Croc material. They sound hideous but go with everything and don't look like kids shoes, they are super comfortable and quick to put on, best trainers I've ever owned in my life! I've not seen any adult trainers with the elastic laces like kids ones yet. Plus I find it easy to find the colours I prefer in older boys sections, navy, burgundy, brown, retro colour blocks etc