So wait, is there a systematic vowel change here? Because the word for ‘political’ changed gwlad into gwleidd-.
The Welsh noun for child is 'Plentyn'
The addition of aidd is a very common way of producing an adjective from a noun, in a similar way to the English 'ly
The addition of the extra 'n' here and in other words is something that is much debated in Welsh orthography but it is the standard at the moment.