Does a UK father need to sign his Thai child's passport renewal?

Yes that is required if your child needs a renewal or new passport.

Yes I have had this experience, you have to go to the passport office and sign the papers in person.

Recently done exactly what you are doing, why wouldn’t you be comfortable in providing a signed copy of your passport bit odd it’s your kids!

Anyway if your not there it will depend on where you are if I Thailand will need to go to amphur office and sign a permission for wife to do passport without you, if abroad need to go to Thai embassy, I must point out I emailed the UK embassy in London and they were reluctant to do it but they relented.

Really you need to be there in person, also I need more information were they born in Thailand or Abroad, because if abroad they will want birth certificates again and old passports all signed by embassy and legalisation in Thailand, luckily we were prepared.

Yes you do

I have to be there to sign when my kids renew.

Unfortunately, requiring signed copies of sensitive documents is a common thing in Thai bureaucracy. If if you hand it out, do it as the smart Thai people do: Use a pen and cross the copy of your document out with a diagonal line. Then sign on top of that line and write the exclusive reason of that copy on the bottom of that line. This way, that copy is way less likely to be misused.

Yes, you either need to show up in person with your kid and the mother (along with your passport) to sign your permission, or you have to send in a permission letter along with a copy of your passport.

See the following infographic by the Consular Department (in Thai):

I know for Thai if they are under 20 they need a special document from both parents giving them permission to get a passport. So I'd assume if your name is on the birth certificate then it's likely true that she would need some kind of document. What that is specifically, I'm not sure.