Source

foreach (var item in searchFilterItems)
{
    if (item.Rows == null || item.Rows.Count <= 0) continue;
    foreach (var child in item.Rows)
    {
        if (child == null || !child.IsSelected) continue;
        var newItem = new SearchConditionItem
        {
            AuditLocationId = item.Id,
            Id = child.Id
        };
        returnList.Add(newItem);
    }
}

Target

returnList.AddRange(from item in searchFilterItems
where item.Rows != null && item.Rows.Count > 0
from child in item.Rows
where child != null && child.IsSelected
select new SearchConditionItem
{
    AuditLocationId = item.Id,
    Id = child.Id
});