If you want to spare the user the trouble of reblessing, do it yourself: make the class the return value should be blessed into an optional second argument to ChildGet().

sub ChildGet {
    my ( $self, $class ) = @_;
    $class = "Etk::Widget" unless defined $class; 
    my $child = $self->{_CHILD} 
        ? $self->{_CHILD}
        : child_get($self);
    return bless $child, $class;
}
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That's functionally equivalent to the C requirement that the user perform the casting. Since the information about what kind of Widget you've got can't be determined from the return value of the C function, if you want to make this DWIMmy Perl and not require that second argument, you're either going to have to add metadata to your C object or go with the global hash. You make the call. :)

You can also do this in XS, if you want.

void
ChildGet(bin, ...)
    Etk_Bin * bin;
PREINIT:
    char * class = "Etk::Widget";
    Etk_Button * button; 
PPCODE:
    if (items > 1) 
        class = SvPV_nolen( ST(1) );
    button = TK_BUTTON(etk_bin_child_get(bin)); 
    ST(0) = sv_newmortal();
    sv_setref_pv(ST(0), class, (void*)button);
    XSRETURN(1);
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I'm not sure that's quite right, because after spelunking your typemap and your .xs file, I still don't grok how you're getting hash objects -- but you get the general idea.

The fact that I'm a half-decent XS hacker and I can't figure out how to do this says something about its maintainability. :)