That is almost certainly Childhood's End by Arthur C Clarke. I won't reveal the ending, since that would be a spoiler, but it's left as an open question whether or not the aliens (known as the Overlords) were truly benevolent.
To quote Wikipedia's description of the first section of the novel:
In the late 20th century, the United States and the Soviet Union are
competing to launch the first spaceship into orbit, for military
purposes. When vast alien spaceships suddenly position themselves
above Earth's principal cities, the space race ceases. After one week,
the aliens announce they are assuming supervision of international
affairs, to prevent humanity's extinction. They become known as the
Overlords. In general, they let humans go on conducting their affairs
in their own way. They overtly interfere only twice: in South Africa,
where sometime before their arrival Apartheid had collapsed and was
replaced with savage persecution of the white minority; and in Spain,
where they put an end to bull fighting. Some humans are suspicious of
the Overlords' benign intent, as they never visibly appear. The
Overlord Karellen, the "Supervisor for Earth," who speaks directly
only to Rikki Stormgren, the Secretary-General of the United Nations,
tells Stormgren that the Overlords will reveal themselves in 50 years,
when humanity will have become used to their presence. Stormgren
smuggles a device onto Karellen's ship in an attempt to see Karellen's
true form. He succeeds, is shocked, and chooses to keep silent.
Their form is one that inspires great consternation.
It was a tribute to the Overlords' psychology, and to their careful years of preparation, that only a few people fainted. Yet there could have been fewer still, anywhere in the world, who did not feel the ancient terror brush for one awful instant against their minds before reason banished it forever.
There was no mistake. The leathery wings, the little horns, the barbed tail-all were there. The most terrible of all legends had come to life, out of the unknown past. Yet now it stood smiling, in ebon majesty, with the sunlight gleaming upon its tremendous body, and with a human child resting trustfully on either arm.
Childhood's End - Read Online