I'm guessing you are using the "AlloyContentAreaRenderer". This override "GetContentAreaItemCssClass".

Alloy version:

        protected override string GetContentAreaItemCssClass(HtmlHelper htmlHelper, ContentAreaItem contentAreaItem)
        {
            var tag = GetContentAreaItemTemplateTag(htmlHelper, contentAreaItem);
            return string.Format("block {0} {1} {2}", GetTypeSpecificCssClasses(contentAreaItem, ContentRepository), GetCssClassForTag(tag), tag);
        }

Base version:

    protected virtual string GetContentAreaItemCssClass(HtmlHelper htmlHelper, ContentAreaItem contentAreaItem)
    {
      return htmlHelper.ViewContext.ViewData["childrencssclass"] as string;
    }

As you can see, the alloy version does not use the childrencssclass. You can add it though, like so (not tested):

            return string.Format("block {0} {1} {2} {3}", GetTypeSpecificCssClasses(contentAreaItem, ContentRepository), GetCssClassForTag(tag), tag, htmlHelper.ViewContext.ViewData["childrencssclass"] as string);

I agree to Per Magnes solution.

I works as expected

Thank you, that was the issue.