Well, for "everyday life" I'd go with either bottom (neutral) or bum (more informal - often used with kids...). In the US however, "fanny" is used quite commonly for these. But it's best not to use this word in the UK, where it has quite a different meaning. (You may check it in your online dictionary...). To be honest I don't find myself using any of these words in everyday life - but maybe if you were a nurse or a doctor... Hope that's helpful...

A safe way to reference that particular area would be "rear end". You could also be creative about it. For example, you could say "caboose". A caboose, unless I am horribly wrong, is an older word that more properly refered to the last car in a train; though we are talking about old style trains, not modern ones. A more recent option is 'tukus', but I've only heard that used when parents are talking with kids. Some people might not get the 'caboose' or 'tukus' options, but pretty much everyone will get the meaning of 'rear end'. There was a rather amusing video that went around on social media that was of a guy talking about the word "ass". The whole joke was that he felt it was one of the hardest words in English to master. It is a bit vulgar, that much is obvious, but it is fairly common. You stand a higher chance of running into it in daily life than some of the more, shall we say, colorful or inventive options. It also refers to a donkey, which may seem a bit random.

Thank you for your kind explanation, John.^-^

In the U.S. the most neutral are probably "backside", "rear end", and "bottom". The word "butt" is less vulgar than ass, and commonly used among friends and family, but less often in public. There are other regional variations such as "hind end", or "hiney". I rarely hear "fanny" used here except among old people, but everyone knows what it means; it may be more common in other regions.

In the U.S. the most neutral are probably "backside", "rear end", and "bottom". The word "butt" is less vulgar than ass, and commonly used among friends and family, but less often in public. There are other regional variations such as "hind end", or "hiney".