"Get me a drink," he barked.
A few minutes later, as I was buying my sons an ice cream, this same lad barged in, tugged at my shirt and demanded: "I want one, too."
'How about please?' I thought, but didn't say.
I asked him what he wanted, handed over the Euros, and gave him just one Cornetto.
He seized the cone with an ungrateful hand, then ran off.
'How about a thank you?' I thought, but didn't say.
I looked at his dad, who had put his hand in his pocket to reimburse me (I refused) and he just smiled, weakly, then raised his eyebrows in a kind of 'Kids will be kids' gesture.
And I accept that.
Children live in the here-and-now. Life is fast, needs are urgent, pleasure is everything. Which is precisely why children need to be socialised – trained – to behave in a manner that makes society a nicer place.
Ice Cream Boy clearly hadn't been taught that please and thank you make life a little more pleasant.
That's the way I was brought up. It's the way I bring up my three kids: if you don't say please and thank you, you don't get the reward (and, when I was a kid, the possibility of a clip round the ear). Simple.
Except Ice Cream Boy DID get his reward – because I was too polite, or too cowardly, to say in front of his dad: "Oi, mister, I'm not your slave. Please is the word you're looking for, or you don't get an ice cream."
The strange thing is that this other dad seemed like a lovely person. Intelligent, funny, considerate. Yet his child was a demanding brat. How so?
My friend Mark has a similar situation with one of his son's mates. This nine-year-old boy's parents are super, smashing, great, but their son is rude and – above all – self-entitled.
On the occasions he's invited round for a playdate, he completely dominates, charges up to my friend's son's bedrooms, tearing into his toy box, chucking things around, bouncing on his bed, kicking holes in the wardrobes and door.
Over tea, he's all 'I want', 'get me this', 'I don't like that', 'this is rubbish', 'why haven't you got?'
"I feel like strangling him," says Mark. "But, of course, that's against the law."
So why do you invite him round? I asked.
"Because for some strange reason my son likes him – and the worst thing is, that my son starts to behave like him whenever he's around."
Of course, some parents don't give a monkey's how their spawn behave, either in private or in public.
They're the kind with ferocious-looking dogs tugging on their leads for all their worth, snapping at the Cockapoos. The kind who play their banging drum 'n bass at all hours of the day and night, regardless of the consequences.
But then there are the ever-so-decent middle classes who believe their preciouses don't have a precocious bone in their spoilt bodies.
So what's a dad, or mum, to do about this situation?
Avoid all contact with the offending children in the first place? That's one solution – but is it fair to cut your kids off from their friends because YOU don't like them?
Talk to their parents about their child's bad manners? Yeah, right. Even if a parent doesn't believe their kid is a butter-wouldn't-melt angel, no parent wants to be preached to about their child's behaviour – because by implication you're calling them a rubbish parent (which, of course, is true).
Deal with the bad mannered child directly, whether within earshot of their mum or dad, or behind closed doors?
This seems to be the most sensible solution, though it raises the question: is it our role as parents to teach good manners to other people's sprogs?
Jenny, 34, a mum-of-three from Manchester, has no qualms about telling off kids who are not her own.
It's my way or the highway. What they do at home is none of my business, but if they're around me and they disrespect me or others, then I will tell them so.
"It's only got me into trouble once: I asked the son of a friend to move from a seat in a soft play centre to make room for a mother.
"The child just said 'No' so I took away his bag of crisps and said I wouldn't give them back until he'd said sorry.
"His mum wasn't amused. She asked me to stop being childish, made me give the crisps back – and then left.
"But I'd do it again. Other parents have said to me: 'I'm glad you told him off – he takes no notice of me at home'."
To me, the 'no notice' argument is a cop-out: if you can't achieve the respect (aka fear of you and your power to inflict punishment) of your children at home, then what hope for them is there in the big wide world?
But then again, perhaps I should just take a 'chill-pill' (as one of my son's friends once said to me, when I told him off for wiping his hands on the furniture) and accept I'm just too old and old-fashioned for this modern world.
What do you think? Let us know...
Do manners still matter?