So, if Obelix fell into the potion as a child, and got the effects permanently without any bad side effects, why don't the Gauls bathe in a large amount of potion? (Or just dip every newborn/chile, for that matter, requiring less potion.) Wouldn't they then all have the effects permanently and not have to worry about Getafix being kidnapped or getting attacked by surprise when not battle ready?
He didn't just fall into the potion, he drank a lot of it while in the cauldron. Also, Obelix's strength turned him clumsy and prone to disaster, so the perpetual effects don't seem that good.
Every single (male) gaul in the village is prone to disaster, and half of them are clumsy, stupid or the best of two worlds. Anyway, Getafix states that if a child drink a large amount of potion, the duration is unknown. For Obelix is permanent; for the son of Caesar, it's a few days. On adults, it's dangerous: a Roman became a statue after drinking to much.
Becoming stone is what happens when you drink a dose of potion while already under its effect, it's what had happened to Obelix when he finally managed to drink some potion earlier in that book.
A recent picture book entry into the franchise, that eleborates on Obelix' bath in the potion as a child suggests, that the huge ammount of potion also drastically altered his personality: Pre-Potion Obelix was shy and a pacifist. So, bathing every newborn in potion would probably result in them having a whole generation of Obelixes to raise.
Also, Obelix eats a LOT. Though not stated, the potion may well cause changes to one's metabolism, to the point in which the sustainability of the village might suffer from a whole generation of Obelixes.
Well, Obélix was fat and, apparently, not too bright even before falling inside the cauldron. Also I don't think the potion changed his personality: he just knew himself invincible and wanted revenge.
One comic showed that drinking too much potion turns people into stone, I believe Obelix drank just enough before he was rescued from the cauldron to give him superstrenght without the side effect, but since it was an accident, maybe the druid doesn't know the exact ammount Obelix drank and doesn't want to risk turning a newborn baby into stone for nothing.
No, it's drinking more potion while still under the effect of the potion is what turns you to stone.
In Asterix and Son, we get to see what an absolute terror a super-strong baby is; even when super-strong themselves Asterix and Obelix can barely manage him, and the property damage is enormous. At least one additional story reveals that children aren't allowed magic potion, probably because they're not ready for that much power yet.
The official website observes that in Obelix All At Sea, when his potion is undone and he is reverted to childhood, he acts surly and precocious and generally more adult than when he is one. It also asks if one of the powers of the potion is to give its drinkers the mind of a child. None of the characters in the comic seem to notice any effects from it, but people on potion do seem to really enjoy beating people up even if they'd been pacifistic or intellectual before (such as the cameos in Asterix in Corsica - and indeed Asterix himself). It's possible the potion does have a psychological effect, but a subtle one. Note the similarity between 'fell in when he was a baby' and 'dropped on the head when he was a baby', after all - and don't forget that the village's star warrior is Asterix, entirely because he's clever. Intellect is much more precious than physical power.
There's also the security issue. If the Romans managed to kidnap a Gaul who was on potion, they couldn't then turn them on their own tribe, because the potion would wear off. A permanently strong person still needs to sleep, and is not immune to pain or certain drugs (it seems they are to sleeping pills, but not booze). Obelix is alright because his sheer size means he couldn't be moved without waking him up and causing consternation, but an insufficiently-guarded child could be.
One Pilote comic showed Goscinny and Uderzo meeting Monsieur Obelisch, a distant descendant of Obelix who shows them his family tree, full of super-strong Man ChildBlood Knight French warriors. Obelisch also has super-strength and uses it to load ships. If this strip is canon, it means super-strength obtained that way isn't just permanent, it is Lamarckian and hereditary forever, along with any personality effects.
This may be just an issue of translation, but in the Druids' competition in the Forest of the Carnutes, one of them demonstrates a potion that makes you immune to pain, and demonstrates this by taking chips out of boiling oil with his bare hands. He seemed to incur no damage, but if pain was the only thing removed, his hands would still have been destroyed by the heat. Did the original French text say "immune to heat"?
The brazillian portuguese version also just said immune to pain. My guess is that they meant immune to pain as in "immune to damage", seeing as in a later comic the village druid comments how the potion gives superstrenght, but not ivulnerability, then adds something to the effect of "but there's other potion for that".
Obelix has a birthday party in Obelix and Co., and no one mentions Asterix. However in a later issue it is revealed both were born at the same day, and always celebrate their birthdays togheter.
I also wondered about that, but it would certainly be in character for Asterix to decide that this year would be for Obelix. After all, Obelix has completely forgotten what day it is, so it's easy to imagine Asterix laughing to himself as he goes around to the villagers and say "Look, Obelix doesn't remember it's our birthday, so I want to arrange a surprise for him."
It was written by Goscinny, and all the instances involving Asterix and Obelix sharing a birthday seem to be found only in albums by Uderzo.