In fact, since you are dealing with bool, you can invert the logic and do it without converters:
<StackPanel Name="sp" />
<Button Content="A Button">
<Button.Style>
<Style TargetType="{x:Type Button}">
<Style.Triggers>
<DataTrigger Binding="{Binding ElementName=sp, Path=Children.Count}" Value="0">
<Setter Property="IsEnabled" Value="False"/>
</DataTrigger>
</Style.Triggers>
</Style>
</Button.Style>
</Button>
, , Children.Count DP, ItemsControl, ( - StackPanel):
<ItemsControl Name="ic" />
<Button Content="A Button">
<Button.Style>
<Style TargetType="{x:Type Button}">
<Style.Triggers>
<DataTrigger Binding="{Binding ElementName=ic, Path=Items.Count}" Value="0">
<Setter Property="IsEnabled" Value="False"/>
</DataTrigger>
</Style.Triggers>
</Style>
</Button.Style>
</Button>