Go XML marshalling - omitted surrouding tag

I have a problem with marshaling struct into XML. I have my structs like this:

type SomeStruct struct {
     Campaigns CampaignsNode `xml:"campaigns"`
}

type CampaignsNode struct {
    First CampaignsGrouping `xml:"first"`
    Second CampaignsGrouping `xml:"second"`
}

type CampaignsGrouping []Campaign

type Campaign struct {
    XMLName   xml.Name `xml:"my_campaign_tag"`
    Id int
    Name string
}

Now, I want to marshal SomeStruct into XML so that it loooks like this:

<SomeStruct>
    <Campaigns>
        <First>
            <my_campaign_tag>...</my_campaign_tag>
            <my_campaign_tag>...</my_campaign_tag>
        </First>
        <Second>
            <my_campaign_tag>...</my_campaign_tag>
            <my_campaign_tag>...</my_campaign_tag>
        </Second>
    </Campaigns>
    </SomeStruct>

But it instead marshals into something like this:

    <SomeStruct>
    <Campaigns>
        <my_campaign_tag>...</my_campaign_tag>
        <my_campaign_tag>...</my_campaign_tag>
        <my_campaign_tag>...</my_campaign_tag>
        <my_campaign_tag>...</my_campaign_tag>
    </Campaigns>
</SomeStruct>

What is going on here and how can I have structure as described above?

Here is example of this behaviour: https://play.golang.org/p/S505JuQ1QGs