You code added most of the XML for a checkbox, but left out the formfield options that tell Word what kind of formfield it is. Your XML:
<w:fldChar w:fldCharType="begin"/>
Corrected XML:
<w:fldChar w:fldCharType="begin">
<w:ffData>
<w:name w:val="Check1"/>
<w:enabled/>
<w:calcOnExit w:val="0"/>
<w:checkBox>
<w:sizeAuto/>
<w:default w:val="0"/>
</w:checkBox>
</w:ffData>
</w:fldChar>