Ok so I found the issue, the property somehow got removed during the move from MegaNavV8 to UmbNav.

I have added it back, and a very quick test seemed to work.

Please update to 1.0.0-rc.2 and the Content property should be back. https://www.nuget.org/packages/Our.Umbraco.UmbNav.Web/1.0.0-rc.2

The code I used for testing:

@foreach (var item in umbNav)
{
    var hideChildren = false;
    if (item.ItemType == UmbNavItemType.Content && item.Content != null && item.Content.HasProperty("hideChildren"))
    {
        hideChildren = item.Content.Value<bool>("hideChildren");
    }
    <a class="nav-link @item.CustomClasses" href="@item.Url()" target="@item.Target" rel="@item.Noopener @item.Noreferrer">@item.Title</a>
    if (!hideChildren)
    {
        foreach (var child in item.Children)
        {
            <a class="nav-link @child.CustomClasses" href="@child.Url()" target="@child.Target" rel="@child.Noopener @child.Noreferrer">@child.Title</a>
        }
    }
}

Please let me know how you get on!

If this solves your issue please mark this topic as solved, thanks!