The solution depends on where you are seeing the URL without /donate/. Since you have a link with just /donate/ on your site, Google will try to index it. If you do not want /donate/ alone to be indexed, all links leading to it need a rel="nofollow" attribute and the /donate/ page needs a noindex meta tag. Better yet, don’t let such an URL appear at all. Rename the /donate/ page slug.
It’s permalink shows page URL correctly:
https://www.littlelives.org.uk/donate/charity-clothes-collection-london/
but slug ignores parent and shows only this
https://www.littlelives.org.uk/charity-clothes-collection-london/
I have a same prob with all the Pages. With posts, slugs show correctly including category
I want the parent to be shown in slug
I’m sorry, but saying the slug ignores the parent doesn’t clarify things for me. In WP terminology, a page’s slug is only the “charity-clothes-collection-london” part, it never includes other object slugs like “donate”. It never includes domain name or connection protocol (https://) either. It’s still not clear to me where you are seeing a link to the child page where its parent is not included in the URL.
When you say the child page’s permalink does include the parent’s slug, it sounds to me like everything is working as it should. None the less, it’s unusual to see a child page link anywhere which does not include its parent, so your desire does seem to be well founded. I’m trying to determine what is generating such links, so knowing where on your site you are seeing it would be helpful.