Can I be honest with you? We have some guinea pigs that we have had from 8 weeks old (the youngest they can legally be sold to you) and some which were rescue whose ages are not exactly known but are older and were full adult weight.
I'm afraid that 3 months after having the ones which we got as youngsters, they were already almost at adult weight (and not through over feeding). Even before then, you can't really see much physical development. Yes they grow a bit bigger but that's it really.
A lot of the physical development I think (from photos I've seen on here of babies) happens within the first 8 weeks of life.
On what I've just said, those with lots more piggies experience may disagree with me and I'm sure they will be able to notice subtle changes but I'm guessing you are like we were and don't have a lot of previous experience with young small animals (other than children - ours were also bought as pets for our children - 9 year old and 6 year old). If you're not really experienced all you are likely to notice is them get a little bit bigger (and not much at that - ours started around 450g if I remember rightly and adult weight is around 900g to 1.2kg).
The best (better than physical growth) thing to watch them develop is their relationship with you as their owner and your children can do that with your help as supervision. When they come to you, wherever they come from and however old they are, they will be nervous. You will get the joy of bringing them out of their shells. I still get a thrill now (as do my children) when it is veggie feeding time - ours have learnt roughly what time of day that happens and start squeaking their heads off if we come near around that time or rustle a bag in the kitchen. We've also got a pair who have progressed from extremely shy to staying right by the feeding bowl when I put my hand in and fill it.
I would strongly recommend avoiding Pets at Home to get pigs from because I've heard so many stories of them getting the sexing wrong and people ending up with pregnant guinea pigs (because a girl has been in with boys or vice versa). Whilst it might sound nice to accidentally end up with some babies this way, I think the reality is often different and the care needed for them could be intensive.
Wherever you decide to get them from though, members on here will be here for you for any questions you may have as you care for them. If to find some piggies somewhere and want to check on here whether it sounds like a safe place to be getting them from or not I'm sure you'll get plenty of views!
