Hi weezy
Please don't worry about buying children's sizes. I am a UK size 4 and often have to try on/buy boots and shoes from the children's sections. There's nothing wrong with them. The only difference I notice is that the soles are softer/more flexible (so wear out quicker but are actually really quite grippy on wet rock). I've found that the actual integrity of the boot is as good as an adult boot (doesn't leak/no issues with stitching, etc). I currently have some boys Gelert (cheap and nasty) boots which have actually exceeded my expectations of them and served me well on some fairly rocky terrain in the Lake District, the Yorkshire Dales, the Peak District, and a couple of hills in the Scottish Highlands. My usual, regular walking haunts are more muddy and grassy fields and these boots are certainly great for that.
Of course, if the majority of your walking is mountainous, rocky terrain then a child's boot won't cut the mustard for regular use due to the flexibility of the sole, but if it's mainly for "general" fell walking with the occasional gnarly thing as a treat then don't be put off buying them.
My only thought is that the sole does wear out quicker than on adult footwear ... but kids boots are cheaper anyway so it kind of offsets that a bit.
Hope that helps.
Mel