to tell a mum she was endangering her baby.

[stubbornhubby]

I was playing petanque on Sunday in a competition in Battersea Park...

petanque = boules = french game, like bowls, played with steel balls about the size of a tennis ball, weigh 0.7kg
these things which are chucked around on a gravelly terrain.

Anyway there were about 150 people playing on 20 marked out areas on a large piste...

...and about 2m from the piste a family were picnicing with a small baby. Small as in unable to sit up. Lying on mat.

After five minutes I spoke to them and told then they really shouldn't have a baby that close to the piste: boule are often 'bombed' where you throw a boule to hit another one, and the boule can shoot off at great speed in all directions. They can easily travel 10-15m off piste., Anyone who has seen this game will know what I mean. If a ball hits an adult on the ankle it hurts. A lot. If it hit a baby....

But the mum of course went ape.

She even came on to the terrain to tell me off. I shrugged, because I was right and she was wrong....

... but perhaps I should have said nothing...

Is it ever right to poke your nose in?

Yanbu, depending on your wording and tone, as others have said. You were just looking out for the safety of the baby.

I always thought it was boules in the north of France and petanque in the south..?

YWNBU.
Don't know how you said it, don't know the tone - but the reasoning behind it was sound and they obviously saw the force of that reasoning if they eventually moved, having made their point that you were being a PITA of course.

I don't understand people who can't see those kinds of danger.

I would imagine she flared up subsequently because your warning was taken as implied criticism that they were being stupid in their choice of where to sit and they werent taking care of their baby properly. YANBU though.

it was a polite but direct criticism :-)