What British word means child or baby?

There may be more than one possible answer here, but the one that comes to mind for me is bairn, which is used both in Scotland and North England to refer to a child.

The Scotsman describes a bit behind the history of the word at http://www.scotsman.com/lifestyle/culture/books/scottish-word-of-the-week-weans-and-bairns-1-3220599 .

The same old Germanic root is still used as the standard word for "child" in the Scandinavian languages: "barn" in Swedish and Norwegian, "börn" in Icelandic and "børn" in Danish. Not in modern (standard) German or Dutch, though.

Infant seems to fit the bill. It has a legally defined cogency as well as common use parlance and woul;d be use across the British Isles.

There are many words for a child or baby in Britain; which one is used depends what part of the country you're in. Kyle's choice of the word bairn is a good one - yes, it's used primarily in Scotland or the North of England but within Scotland it's more common in the East. In the West of Scotland, the word used is more likely to be 'wean' or 'wee yin' meaning 'little one' (the opposite of Sir Billy Connolly who is known as the 'Big Yin', the big one).

Down in Wiltshire where I grew up, a generic term for a child was 'babber' but there were also sex-specific terms. Girls are often referred to as 'maids' and our next door neighbour would address me as 'little maidie' - as I grew up, he dropped the word 'little'.

In Southern England, children are often referred to as 'nippers'. It can be used simply as a word meaning child "how many nippers have they got now?" or in a slightly derogatory sense, possibly stemming from the word 'nip' meaning steal - nipping was pickpocketing and that was mostly done by children. Nipping also means moving fast; children do that too.

http://www.scotsman.com/lifestyle/culture/books/scottish-word-of-the-week-weans-and-bairns-1-3220599
http://www.somersetlive.co.uk/news/history/23-best-west-country-words-45204
https://en.oxforddictionaries.com/definition/nipper
https://en.wiktionary.org/wiki/nipper