I remember having the same concern when my children did Core B. What is helpful to keep in mind is that we, as adults, are often more concerned about historically appropriate nudity than our children are. Kids will take their cue from us ... if we make a big deal of it, then they will believe there is reason to be embarrassed or concerned.
That being said ... a couple of suggestions that you might find helpful ... you can go through the few pages that show under-dressed people and using a black Sharpie, you can "dress" them before you allow your students to see the book. You can also choose to leave them "as is", and work through the pages without any additional commentary. If your children notice or ask about the under-dressed people, you can explain that in that time period it was very "normal" for people to dress scantily. Talk about why ... and then move on. If they seem bothered by the lack of clothing, then feel free to help them use a Sharpie to dress them more like we would expect today.
Ultimately, it will be your personal choice on whether or not you use the Usborne titles. We do believe that they are an excellent resource for teaching World History at this age level. However, if they are not a good choice for your family, then you are certainly welcome to set them aside. You will still have a solid History study with the other titles in the Core.
(referring to the nudity in the Core B Usborne titles) 2.14.14 jaw