He told me the gist of it later: the child had flat feet which would need corrective treatment. We shouldn't neglect the problem: it could only get worse.
Back in England, my partner occasionally nagged at me to take him to a doctor but I found the whole thing hard to take seriously. True, his feet did tend to roll inwards, but did it matter?
About two years later, when Harry was well past the toddler stage, we both noticed he had developed quite pronounced knock knees. While the upper part of his legs curved together, the lower part splayed outwards. My partner, increasingly anxious, said the knees were clearly connected to the flat feet and we had to do something. I caved in and took him to the GP.
Eventually, we were referred to a nice young consultant in paediatric ortho-paedics who apologised for the hour's wait, examined Harry's feet, watched him walk - and then bent his thumbs almost backwards. Our son, apparently, had slack, or double joints - hence the thumb trick. This caused the feet to roll inwards and, probably leading on from this, the legs to curve together.
Knock-knees are so common between the age of three and five years that they are considered part of normal development, he told me. They are caused by the curving of the upper tibia, part of the shin just below the knee joint. Whereas at the toddler stage the legs tend to bow, with more growth occurring on the outer part of the tibia, the bone compensates for this by curving in the other direction a little later. But in most cases, sooner or later, the body gets it right. Knock-knees and flat feet tend to go together, although no one is quite sure which comes first.
In England, doctors tend to leave knock-knees alone, arguing that most children grow out of the condition, although surgery may be needed if they haven't done so by the age of 11 or 12. But in the US and parts of Europe,they tend to go in for a more active approach (which explained the German grandmother's reaction): exercise to develop fallen arches, supportive shoes and even special braces and casts to correct the legs.
According to our consultant, there is no evidence that any such measures make any difference. But if it made us - or the German relatives - happier, he could prescribe corrective heel cups; they were rarely used these days but at the least they wouldn't do any harm. I gratefully accepted the offer.
Our next stop, prescription form in hand, was Messrs H. Clogg Ltd, a tiny shoe clinic next to the hospital which smelt strongly of shoe polish. There Harry was measured for a pair of plastic heel cups which fitted into the back of his shoe and which he wore continually for the next few months. My partner and I, meanwhile, began to stare obsessively at his legs to see if they were getting any straighter.
One day in the playground Harry took his shoes off to make his journey down the giant slide even faster. Half an hour later, when we went to retrieve the shoes, one heel cup had disappeared. There followed another appointment with Messrs Clogg, but the second pair of heel caps rubbed his feet. Sheepishly, we gave up on corrective measures, deciding to let nature take its course.
Harry is now five and a half. His feet still roll inwards and probably always will, but his knees seem to curve together less than in the past. It's unlikely he'll turn into a knock-kneed adult.
Last Easter, we took him on another visit to the Baltic after an interval of three years. The German grandmother was so delighted to see him that all mention of flat feet was forgotten.