After many hours of bashing my head against the wall, I came up with an answer using ConvertAll<T>

If you want to use lambda expressions instead of the dynamic linq strings, you have to convert the List<DynamicNode> into List<dynamic>

//Declare the Items as a DynamicNode list
List<DynamicNode> children = @Model.Children.Items;

//Declare the dynamicList type so the lambda knows that the delegate is a dynamic object
List<dynamic> dynamicList = children
//Cast the list of DynamicNode objects to dynamic objects by passing in the convertion 
//function into ConvertAll
.ConvertAll<dynamic>(delegate(DynamicNode x) { return (dynamic)x; });

 //Use your lambdas! (how do you pluralize lambda?)
dynamicList = dynamicList.Where(x => x.location.cityid == sumNumber).ToList();

If someone else has a better idea I'll look forward to it :)