How much time needs to be remaining on a UK (under 5) child's passport for travel from the UK to Portugal?

According to the law (D.L. 59/93 from March, 3rd), all foreigners need to have a passport that is valid until after the date the person returns to his/her country.

There are many exceptions to the rule, mainly about the countries who have different type of agreements with Portugal or are entitled to a "laissez passer", or have the document mentioned in the Genevra convention (July, 28th, 1951).

So I'm missing data here...

The best you can do is check this in the Portuguese Embassy near you. I'm sure they will inform you asap.

I'd ring the passport office and ask them. I went to Australia at Christmas with only 4 months remaining on my passport when I returned. I rang the passport office before I left and they told me I could use it to come back to the UK as long as it was still in date on my return. Don't know if it's the same for kids.

It is 3 months, and this is added on to the term of you new one

At least 6 full months I think