I have 3 piggies and a 3 year old girl, we got the piggies 2 months before she turned 3. Like you I was concerned because she was so young and it would be very unfair and unrealistic to even try to make her be quiet all the time, but I have to say all my concerns have been proved wrong. Although I never leave them all together unattended just in case.
My advice would be to explain to your little boy that for the first few days the piggies will be very scared, just like he would be if he were in a strange place without his Mama, and that everyone in the house has to be a little more quite and kind to them until they learn that they have come to a good home, that is how I explained things to my daughter.
After a few days I would just let things get back to normal, I have noticed that my piggies don't mind my daughter's loud playing at all, but they do seem to be upset by angry voices even if they are not that loud.
Also I would advise lots of hidey places in the cage for the piggies for the first while, it makes them more confident about exploring if they know that they have a safe place to scoot into. Later you can remove them to give the piggies more room to run around when they've got more confidence. Don't worry about buying the extra hidey's that would just be a waste of money, because their favorite one will be a cardboard box with a couple of holes cut in it, or a piece of cloth tied like a tent across the corner of the cage. The placing of the holes in the cardboard boxes is something that your son can get involved with, my daughter loves doing that with me and now she is always on the look out for good boxes for the piggies when we go shopping, she has really become inventive with it, last time we did it she got me to cut round holes in two boxes and then she set about joining them together with a piece of drain pipe that the piggies use as a tunnel
You do need to be extra careful during floor time as it would be very easy for a piggie to get stood on by a small person who just wants to get from A to B to get or see something that has caught their eye, and they have forgot that the piggies are in between, I made it a rule that you could only be in the piggies floor time area if you were sitting down and that went for everyone, adults too! Actually I found that the piggies responded really well to my little girl, possibly even better than to me, I think she was so much smaller, and maybe they saw her as less of a threat, I really don't know, although she really does have a most unusual ability to sit still for a 3 year old, but now if she gets in and lies down they will all come over and climb on her and sniff her face and it has to be said nibble her hair rolleyes, I do keep a very close eye all the time just in case of accidents, and she knows to ask before sitting up and I am there to keep watch and tell her if there is a piggie near her feet that needs to be moved before she can sit up, and when it is my turn to get in with them and be climbed like Mount St Mommy, she keeps a look out for me!
One other thing that we did together for months before the piggies even arrived, was read guinea pig books, mostly for adults, but they almost always have lots of pictures, I know that lots of people dismiss the general guinea pig books as at best rubbish and at worst dangerous rubbish, and mostly I agree, but for getting a three year old who was very used to sharing all her food with her best friend (our dog) to understand why she can't share all kinds of food with her new friends, not to mention all the other little bits of information she picked up, I found the books brilliant, we read them every night in bed instead of bedtime stories, and even now she will tell anyone visiting for the first time how important it is that guinea pigs only have the right kinds of food that they are herbivores

.
All I can hope is that your piggies bring you and your family as much joy as ours bring us, we count ourselves very lucky indeed that these wonderful little creatures entered our lives.