To think this isn't going to work (passports)?

[Panamii]

DH in charge of renewing kids passports. Missed that DS's British passport expires while we are away. DS is a trinational (American/Eu country/U.K). DH reckons he will get in to the US on his valid American passport and back into the U.K. on his EU passport? Will this even work? So stressed and due to leave tomorrow!

You are meant to enter a country on the passport of that country, if you have one. So entering US on US passport, and UK on UK passport. They will let you into the UK on an expired passport no problem*, its other countries ttheyhave to be valid for.

*they might be a bit sniffy and tell you to update it but legally you're allowed in.

Also, better to enter on expired UK passport than EU as if he enters on EU, it will register as overstaying unless he leaves the country again on that passport within 90 days

He needs to enter the UK with his UK passport.

Just like you have to enter the USA with an American passport if you have one.

Oh anther thing- when you check in use an in-date passport, or the airline won't let him fly.

@Justcannot When we check in for the outbound flight we will use his American passport which is valid. It's the return to the U.K. flight that's the issue.

I think you'll be fine. Fly into the US on the US passport and back on either the US or EU one, but take his expired UK passport to prove that DS is also a British citizen.

Yes - you legally have to enter the US on a US passport if you are a US citizen

Only some countries require you travel on their passport of you hold nationality in that country - the US is one, the UK isn't (I'm pretty sure!). So you should use your kid's US passport to go to the US, but it doesn't really matter which you use to go back into the UK.

If you show the expired UK one as well as the other valid one, they shouldn't stamp the other one, but even if they do, it doesn't count because he has the right to live in the UK regardless. (In the same way that some over-zealous EU border guards stamp passports of UK citizens living in the EU when they go home to their country of residence after traveling to the UK - it doesn't start the 90-day clock ticking, it's just an error.)