Evolutionary conditioning for sweet tooth
Evolutionarily, indeed sweet foods have offered an advantage to their consumers: lots of calories. Almost all of us - not just infants, but most adults, in fact, most animals too - are attracted by sweet flavours because of this. But especially us humans, since our big brains crave a lot of energy. And even more so for children, who require an immense amount of energy for their growth, and the tremendous amount of physical and mental (learning) activity they perform daily. It is so much easier to get one's needed energy intake from e.g. honey or fruits than from raw vegetables or meat, much less from grass.
As you note, for a long long time sugar was not available in its pure form, the closest to that used to be honey (or maybe maple syrup in some locations). It was a rare treat, not an everyday indulgence, thus in nature there was no risk of overeating it. Nowadays there is, but we don't have a built-in protection against it. It is now widely known that it can cause tooth decay, also obesity, and subsequently heart disease and other illnesses, but there may be other issues too.
Nutritional value (or lack thereof)
I am not a medical expert, so this is only my subjective take about the issue. In its raw form, be it in honey, cane or fruits, there are lots of other useful stuff - vitamins, minerals etc. - in the food apart from sugar. These provide nutritional value, and help absorbing and digesting sugar. I have read that digesting sugar actually requires vitamin B2, which normally is present in the honey / fruit, but not in refined sugar. So not only is refined sugar absent of any nutritional value, but consuming it in fact depletes your vitamin resources further. So it is advisable to limit sugar intake, and replace refined sugar with (fresh or dried) fruits, raw cane / demerara sugar etc.
Blood sugar level and mental / energy state
There are also lots of different kinds of sugars and sugarlike materials (e.g. starch). Some are absorbed faster, some slower. When sugar is digested, it gets into the blood, raising blood sugar level. In natural food, sugar is absorbed slower since its concentration is lower, it is surrounded by lots of other nutrients and it may also need to be transformed first into another form of sugar (glucose) which is directly usable for our body. Thus blood sugar level rises gently, and since digestion takes longer, it is sustained fairly steadily for a longer period of time. Higher blood sugar levels make you active, energetic and positive. When the sugar level starts to decline, you get hungry again - and also tired, and potentially angry or in a bad mood - and the cycle repeats. Refined sugar, however, is absorbed much faster, so it kicks up blood sugar levels faster and to higher levels, potentially making one overly agitated. Soon after that, blood sugar level drops, since there is no steady supply, potentially causing a sharp mood swing towards fatigue and depression. That's when a lot of us reach for the next candy bar, to revive and repeat the cycle...
Hypoglycemia and diabetes
My wife has a condition commonly called hypoglycemia. She is very sensitive to variations in her blood sugar level, thus absolutely can't fast, and must get regular meals in about every 3 hours, otherwise she turns into a dragon. She had suffered from an almost manic-depressive intensity of the ups and downs described above, for many years, before she somehow happened to realize it was caused by sugar. Since then, she more or less successfully restricts her intake of refined sugar (fruit sugar is OK), keeping her mood swings at bay.
According to the book Sugar Blues, hypoglycemia is actually pretty common, just most people never actually realise it is caused by refined sugar. The book also claims that it can turn into diabetes proper if left unnoticed for years or decades. I think the book contains some pretty extreme opinions which I don't identify with (like linking sugar with bubonic plague), however I think there is at least a grain of truth in many of its statements. E.g. as per my wife's story, I can see how in some extreme cases sugar may cause symptoms (mis)diagnosed as a mental illness. Also, its explanation of how prolonged repetition of the above high-low blood sugar cycle may eventually wear out the pancreas so much that it stops producing insulin, resulting in (type 1) diabetes, sounds plausible to me. I would be very interested to hear scientific opinion about these claims.
Effects on children
Children are typically more sensitive to such effects, and in my personal experience, refined sugar intake can have dramatic effects on them. On our own (and others') children, we have regularly observed hyperactivity and sometimes very difficult behaviour after taking lots of sweets (in birthday parties etc.), then intense fatigue or hysteric breakdown after an hour or two. So we try to reduce their sugar intake towards the socially acceptable minimum (without being overzealous). IIRC they got almost no refined sugar below 1y, and still not much after that (except at birthday parties etc.) until they started to eat the same food as us adults.
This is only our own subjective experience though, and AFAIK there are no scientific studies to prove this effect (or at least, the scientists conducting such studies may have been singles without children :-).