Scotland or Ireland with elementary aged children?

I would pick Scotland. I’ve been to both Scotland and Ireland in March and had no rain in either place, so hopefully you get lucky too! Edinburgh is a beautiful city, with museums, the castle, Royal Mike, tons of character. Scotland is more rugged than Ireland, in my mind. Ireland certainly has its wild areas, but much of it has been carefully cultivated over time. The Highlands, especially Skye, will give you some gorgeous areas to explore. Scotland also felt more “foreign” to me, but that may be because we had some trouble understanding very thick accents! Part of the fun!

The Scotland Explorer pass might be helpful: https://www.historicenvironment.scot/visit-a-place/explorer-passes/

I’m going to be honest about the scenery in Scotland. It is beautiful, but if you are from the PNW, it may not have the same impact on you that it might for other people from different parts of the country. It all seemed somewhat familiar in a toned down way to me.

There are a good number of castles, museums and genuinely fun places to explore. For family fun, I would suggest playing the Himalayas putting course at St Andrews, The 3 island tour (with puffin watching) out of Oban, The Falkirk Wheel (excellent interactive science based playground there as well) and maybe dipping down a bit into England for Hadrian’s Wall. Your history buff would be amazed by the Roman ruins and artifacts there. If you make it down to Hadrians’s Wall, consider hiring Peter Carney (retired history teacher) as your guide for the day. He will make Roman History in Britain come to life for your children. Walking a section of Hadrian’s Wall for a day sounds like a good match for your family.

Another vote for Wales. It’s easier to see the sights of north Wales rather which aren’t spread over as large an area than Scotland.

Plenty of castles, walk over the stunning Pontcysyllte aqueduct near Llangollen and take a trip on a horse pulled boat on the canal, lots of National Trust properties, great hiking around Snowdonia, canoe on Lake Bala plus Liverpool and Chester are nearby.

In Ireland, there is incredible history but the castles aren't great, most are fortified homes (or manor estates) rather than what someone pictures as a castle. They're all over, but few are worth visiting. I'd suggest Bunratty as a good choice.The best history IMO is seen by visiting prehistory sights such as Bru na Boinne and early monastic settlements like Glendalough and Clonmacnoise. The kids would probably love a day or two on Inishmore to see the round stone fortresses and tiny monasteries/oratories that date back to around Roman times. Skellig Michael would be awesome but may be too much for kids their age.

I love both Ireland and Scotland, but agree with other posters that Wales has more castles per capita than either.
All are scenic, but both Ireland and Scotland are larger and take more time to see.

North Wales has Conwy while we loved Tenby in the south. Cardiff has its castle as well as others nearby and the Museum of Welsh Life (outdoor museum).
Between Ireland and Scotland, I would go with Scotland. Just going to Edinburgh and Lock Ness in the north were wonderful.
Of course, the Ring of Kerry is awesome.

Haven't done Ireland yet, but my husband and I took our kids (ages 11, 8, 6) to England, Scotland, and Paris for three weeks this past summer for their first international experience. :)

Loved Edinburgh! Such a great feeling in the city, people were so friendly, visited Edinburgh castle... my oldest loves Harry Potter, so she really enjoyed how JK Rowling got inspirations for the books while in the city. We even visited the cemetery that has many of the Harry Potter characters names on the old tombstones, where she got some ideas. My kids also enjoyed the story of Greyfriar's Bobby. Just a really fun experience!

A two-hour train ride south will bring you to York, England - my kids also loved this Roman/Viking/medieval town. Also a great train museum - my 6 year old enjoys transportation!

Enjoy!