Children's party entertainers are worth their weight in chocolate cake. Take train driver Harry, the lad booked for my rail-mad grandson Little L’s fourth birthday bash on London’s Highbury Fields. He’s not a real train driver, at the dead man’s handle on the 0900 to Newcastle.

He pretends to be a driver in old school blue cotton shirt and black dungarees – no longer industry uniform. Grandpa Jake, another of Little L’s grandies, raised the absence of train driver Harry’s locomotive. The local council wouldn’t allow tracks across one of the capital’s finest beauty spots.

Not that that was the issue. Harry’s an entertainer. He doesn’t have a train.

End of. Granny Vanessa shushed giggling Grandpa Jake to avoid the entertainment hitting the buffers before it left the station. Hers was a fair point.

I was too busy being the Fat Controller, signalling beers for parents, to catch his patter but it must’ve been convincing because for the best part of an hour, he had 16 kids enthralled. Sitting on the grass listening intently to whatever yarns he spun. Running excitedly around the park in a human train.

Laughing ecstatically the way kids do, the world’s sweetest sound. But kids’ entertainers aren’t really for children. They’re primarily for us adults.

While train driver Harry gathered speed, whisking children away on a magical journey, we parents and grandparents made merry. Kids are tickets to drink, eat and gossip without guilt at their birthday part.