Family Christmas vacation

Planning a family Christmas / New Years vacation. Very adventurous family of five. (Kids 9, 11, 13). I posted on a few groups on Facebook, but the only replies or comments I get are bot replies just trying to push a book on Amazon. Tentative plan is to arrive in the morning on December 27th from the US. Stay a few days in a southern region where we can visit ice caves, black beaches, take in northern lights, etc. Then to stay a few nights over New Years in the city itself (Reykjavik) to catch New Years and leftover Christmas celebrations, see the fireworks on New Years, visit the Blue Lagoon from there, and pretty much everything the city has to offer. Now for specific ideas. Places to see, things to do, places to eat. I know many of the restaurants need to be booked months and months in advance. Any help is greatly appreciated. Or if I need to change my entire scheduling, please advise. Thank you