Informally, in British English:
– the nouns lollipop, lollipop sign, etc., denote a circular sign on a pole held up to stop traffic so that children may safely cross the road near a school;
– the nouns lollipop woman, lollipop man, etc., denote a person who helps children cross the road by holding up such a sign.

The official name of a person thus employed is school crossing patrol, as specified by the School Crossing Patrols Act, 1953: An Act to provide for the authorisation of measures for the control of traffic, at places where children cross roads on their way to or from school, by persons other than police constables:

When between the hours of eight in the morning and half-past five in the afternoon a vehicle is approaching a place in a road where children on their way to or from school are crossing or seeking to cross the road, a school crossing patrol shall […] have power, by exhibiting a prescribed sign, to require the person driving or propelling the vehicle to stop it.

The noun lollipop man was one of the Briticisms in a “little quiz from the friend in Merrie England” that Mary Charlesworth Shadle published in her column Around Town, in the West Schuylkill Herald (Tower City, Pennsylvania) of Thursday 7^th February 1974:

Since the pound is down and the dollar up in England, we read, perhaps some of us would love to travel to that part of the universe. If we can defeine [sic] these words, she [= the English friend] says, at least eight of them, we can do it alone; if not, she knows we’ll need a little assistance.
Biscuit is cookies; jumper, sweater; pavement, sidewalk; lollipop man, school crossing guard; petrol, gasoline; subway, underground walkway; potato chips, French fries; nappies, diapers; the loo, the toilet; Father Christmas, Santa Claus.

The earliest occurrence that I have found of lollipop used in reference to school crossing is from From The Council Minutes, in The Motherwell Times and General Advertiser (Motherwell, Lanarkshire, Scotland) of Friday 5^th April 1957:

A school crossing patrol officer—or a lollipop man as some school children call him—is to be provided to ensure the safety of pupils crossing to and from the temporary classrooms in Park Street. One of the “School” road signs is to be re-sited in Park Street as a further measure of safety.

The second-earliest occurrence that I have found is from Girl (16) is menace on road, published in the Peterborough Citizen and Advertiser (Peterborough, Northamptonshire, England) of Tuesday 16^th July 1957:

A 16-year-old Whittlesey girl was fined £3 by Whittlesey Juvenile Magistrates yesterday because she failed to halt on a motor scooter when she was signalled to do so by a school crossing patrol warden. […]
[…]
Joan Lilian Larham, of 10 Syers Lane, Whittlesey, the traffic warden on duty, said that she went into the middle of the road and displayed her usual “lollipop” traffic sign asking traffic to stop: she began to lead some children across the road.
When the children were half way across the road, the girl, who was a learner driver, came along Broad Street and went past the children while they were crossing.

The expression the Lollipop Brigade occurs in Children from Ulster score in road safety competition, published in the Belfast Telegraph (Belfast, County Antrim, Northern Ireland) of Friday 22^nd August 1958:

Children in Belfast are strongly road safety conscious, judging by the standard of their entries in a nation-wide competition run by the 45,000-strong S.O.S. Children’s Road Safety Club.
[…]
[…] 11-year-old Mavis Scott […] was one of 158—against 47—children who gave a vote of confidence to the “Lollipop Brigade”—school traffic wardens with their lollipop-like signs. The wardens have been criticised in some quarters as being too elderly to carry out their work efficiently, but Mavis said: “In my opinion older people are more experienced with children and should be able to carry out their duties more efficiently.”

In a letter published in the West Lothian Courier (Bathgate, West Lothian, Scotland) of Friday 26^th December 1958, one Robert Ferguson described the school crossing patrol as:

rather a conspicuous figure in his uniform of white coat, peaked cap and the “lollipop” pole.

This informal use of lollipop has been borrowed into Australian English. For example, the following is from The Sydney Morning Herald (Sydney, New South Wales) of Friday 9^th May 1975:

Lollipop men will guard school crossings

The Minister for Police and Services, Mr Waddy, explains to Andrea Nopper, of Leichhardt, how the new school-crossing patrols will work. With them at the demonstration in Bent Street, City, yesterday are two supervisors, Mrs Junette Stearman, of Coogee, and Mr Alex Bunt, of Mascot.

Police released to other duties

Sydney schoolchildren will soon have their own lollipop men.
Wearing bright, safety-yellow jackets and holding red and white Stop signs, the lollipop men (and women) will supervise difficult pedestrian crossings outside schools in the morning and the afternoon.
They will relieve some 70 policemen on duty at crossings in the metropolitan area within the next three weeks.
The lollipop scheme was given the “green light” by the Premier, Mr Lewis, earlier this week after a successful three-month trial in the Eastern Suburbs last year.
It will cost the Government $95,200 a year for the 70 crossings but it is hoped to extend this as more funds become available.
Announcing this yesterday, the Minister for Police and Services, Mr Waddy, said he expected that many of the part-time supervisors would be housewives and pensioners. They will be paid $30 a week.
Mr Waddy said the supervisors would have no power to take action against offending motorists but could report any incident to the police.