My friend is trying so hard to fit into polite society, and is raising her child to say loo rather than toilet. I know it should be lavatory (and I would not say lav) but we are in the 21st century now. What would be deemed appropriate?
In the UK, people would be happy if you say loo.
An alternative to loo is lavatory, which is something you might hear in higher class circles.
Toilet is fine, but will make some, more old fashioned, people wince.
If you say "bathroom" (as in MrHen's answer) in the UK, people will probably understand you, but if you are out and about (rather than in someone's home) then they might be a little confused.
We don't tend to use "restroom" in the UK, but, again, most people will understand you.
Other euphemisms for toilet in the UK, that are used in polite society, include
- Gents (as in gentlemen's)
- Little boys' room
- Little girls' room
- WC (water closet)
and somethings I've only heard from my granddad's generation:
The terms used for the room-sized location where human waste is expelled tends to be highly regional.
In the US, the term "toilet" is almost exclusively reserved for the plumbing apparatus rather than the room where it is installed. Because in most American home construction the toilet is located in the same room as the bath tub, when a person asks for access to another person's toilet in a polite way they ask "May I use your bathroom?" It's common knowledge in the US that such a question indicates that the asker wants to use the toilet. The term "restroom" also seems to be an American term that is not commonly used elsewhere. My Dutch aunt was appalled the first time she experienced a public "restroom" in a gas station. She had pictured a room where you could actually have a restful experience, perhaps with a couch or comfortable chairs. In most places in the US, although people know what a "lavatory" is, that term sounds very technical and is usually reserved for airplanes. Another generic euphemism that is common in the US is "washroom". "Washroom" may be the most widespread euphemism that does not have the possibly misleading connotations that "bathroom" and "restroom" have. In the US, I have only heard the term "latrine" used in military TV shows or by ex-military co-workers. To me, that term paints the same foul picture I associate with "outhouse."
In Europe, and other locations where the toilet fixture is not installed in the same room as the bath, the term "bathroom" is completely foreign. In these places the terms "water closet", or simply "WC", "lavatory", or "loo" are the more common euphemisms for needing to use the toilet. Of course, outside the US, it seems that it is not completely uncommon, nor impolite, to ask "Where is the toilet?" or "May I use your toilet?"
MrHen is correct, much depends on location. It also depends on the child's age; as adults we understand that "loo", "lavatory" and "toilet" are all the same thing, but for young children it is usually more helpful, for them and for those around them, to use language consistently. Therefore I would encourage parents to use the more common word ("toilet" in the UK) as this is what the child is more likely to hear other people saying (including other children, teachers, and other carers).
Making sure children can understand what's being said to them, and make themselves understood, is more important than adhering to some archaic code of etiquette. The archaic nature of the idea that "toilet" is somehow impolite is evidenced by the emergence of signs saying "public toilets" instead of "public conveniences"
Slightly more formal terms in the U.S. are men's room and ladies' room.
Variants of those are little boys' room and little girls' room. I would never expect to hear one man ask another "Which way to the little boys' room?", but it would not be too surprising to hear one woman ask her friend "Which way to the little girls' room?".
To give another perspective: whereas reasonably crude terms will make some people cringe, terms like "loo", "little boys'/girls' room" etc. will make others cringe. The term "loo" was simply not used where I grew up, and to me it feels very prissy. It's all rather context-dependent; I know people who would, amongst close groups of friends, simply announce "I'm going for a shit". That might be extreme, but it perhaps serves to illustrate quite how out of place "I'm just off to powder my nose / spend a penny / use the little boys'/girls room", or even just "I'm going to the loo", might sound in some cases.
Anyway, I'd say distance is the polite thing. There's nothing impolite about the word "toilet" if you want to use a word for it, but in more formal environments, you might want to use the term "bathroom" (which, contrary to what Matt Ellen said, I believe is understood even when there's no hope of a bath nearby!), or " gents' / ladies' " (this has a sort of 'casual-but-professional' feel to it to me). But by far the most context-neutral, polite and distant way to go about things, at least when you're out (e.g. in a restaurant), is simply to excuse yourself. This has politeness issues too, but they're much easier to deal with: the difference between "I'll be right back" (casual) and "please excuse me for a minute" (formal) is pretty small.
Both lavatory and loo are fine, and it's meaningless to talk about which is correct or more correct, IMHO. Interestingly, these terms are quite strong class indicators in the UK: loo is more often used by middle class speakers than, for instance, toilet. When I was young, I once mortified my parents by asking a family friend if I could use their bog.
Not sure whether this is relevant, but a teacher of mine (female, late middle-age and Irish) became very irritated when someone in her class used the term 'loo' as opposed to toilet. Nobody understood why, but since she was widely regarded to be a lunatic, and we were quite young and timid at the time, nobody questioned it. I've never heard this reaction from anybody since. Anyone able to shed any light on this?