"Pardon? You were speaking too fast."
Translation:B' àill leibh? Bha thu a' bruidhinn ro luath.
I think I read somewhere that some people have started using B' àill leibh? (just this specific expression) regardless of who they are speaking to, as if the actual meaning of the words has been forgotten. However I have never heard this, and it is certainly not true that you can mix the singular (thu) and plural (sibh) forms in general, even in situations where either on its own would be acceptable. It is difficult to think of a similar example in English. Perhaps in the Six Nations Rugby,
Wales has won its match but England have lost theirs.
Whilst you can argue about whether teams are singular or plural, you cannot change your mind half-way through the sentence.
I am not sure what you mean by 'children'. You would never use the sibh form for one child, but you would always use it for children as with any plural addressee. D
Àill means 'desire, wish' so [am] b[u] àill leibh? means 'would desire be with you?'. It seems to be the same idea as French s'il vous plâit, literally 'if it is pleasing to you', but usually translated as 'please'.
Mark (2003) says
àill nf indec will, desire, pleasure most often used with assertive verb in the following way: thuirt e gun dèanadh i mar a b'àill leis air a' cheann thall he said that she would do as he wanted in the long run □ bha e làn di-beathte tadhal nuair a b'àill leis he was heartily welcome to visit when he liked □ b'àill leibh? (for dè b'àill leibh?) pardon? / excuse me (I didn't hear)