De or het zeehondje?

The correct Dutch article is:

het zeehondje

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

How to use zeehondje

the zeehondjehet zeehondje
this zeehondjedit zeehondje
that zeehondjedat zeehondje
our zeehondjeons zeehondje
your zeehondjejouw zeehondje
every zeehondjeelk zeehondje
which zeehondje?welk zeehondje?
a big zeehondje / the big zeehondjeeen groot zeehondje, het grote zeehondje