Halong Bay vs. Ninh Binh?

We need some travel advice please. We're a family with kids age 7 and 12, visiting in January for ~12 days. We are tight on time and budget so gotta make some tough choices, can't go everywhere and do everything.

From Hanoi, our plan was to head to Halong Bay but ngl the price tag has scared me off; $1.2k for 2 nights is wildly beyond our usual excursion budget. And I'm concerned that since it'll be early Jan. it could be too cold to enjoy fully (are we really gonna kayak in January?), foggy obstructing views etc.

I've since read nice things about Ninh Binh. Should we swap Halong Bay for Ninh Binh instead? Or try to do both?

We'll be continuing from Ninh Binh by train (doing the full train route from Hanoi to Saigon) so the convenience there (vs. Halong Bay) is also an incentive. Thanks in advance!