De or het knuffelhondje?

The correct Dutch article is:

het knuffelhondje

Rule Diminutives (-je, -tje, -pje): always het. See the rule

How to use knuffelhondje

the knuffelhondjehet knuffelhondje
this knuffelhondjedit knuffelhondje
that knuffelhondjedat knuffelhondje
our knuffelhondjeons knuffelhondje
your knuffelhondjejouw knuffelhondje
every knuffelhondjeelk knuffelhondje
which knuffelhondje?welk knuffelhondje?
a big knuffelhondje / the big knuffelhondjeeen groot knuffelhondje, het grote knuffelhondje