Mary,
We elected not to take our then 3 year old for a variety of reasons; some being health based/concerns.
If I were traveling with my children I'd have them recieve the same Hep A and B shots we got (and make sure they're 'up to date' on their innoculations).
China's air quality is much much worse than the US. Depending on the province your child is in one can have difficulties breathing/ get colds/ have allergies etc.
If one travels in the summertime one would be concerned for mosquitos and prevention of bug bites.
Although hotels are fully air conditioned most other buildings one goes to aren't. That means gov't buildings aren't; many stores and restuarants aren't. So in the summer it's HOT!!! Also you're often walking up and down stairs to offices (granted this is only for gov't appts).
Water and food: you will have to make sure your sons don't drink any tap water including water when they're in the bath tub. The food is different and one doesn't (or shouldn't) eat any fruit or veggies that does not have a peel to it (you don't eat the peels...).
Again, depending on where your child is from you may find American style foods or restuarants.
Even American style foods will often taste differently. The milk doesn't taste the same, meats don't etc. For some kids this isn't at all a problem; for others it can present challenges.
Availability of doctors: Medicine and medical care is different in China. Yes, your reps will get you any and all medical attention you need but it's not the same as here in the states. And the medical facilities are different between provinces.
If I were to take small children I'd have another person come along to help out on gov't appt days and so there's another 'room' for the kids to sleep in or play in in case the new child is having great difficulty sleeping. Also it's another pair of hands with strollers etc.
I'd also talk at length with our ped about precautions, meds to take with us etc.
Many families travel successfully with young children. Some do so without another family member/ adult friend with them. Some folks do great with the air quality, food differences etc and the kids are travel champs!!!
And then there are toddlers/ preschoolers who have trouble with the time changes, all the flights, different hotel rooms, different foods etc.
We had a few small children on our last trip. Some did great, one was absolutely miserable the whole time
The only people who can really decide what's best for your family is you and your husband. You know your boys, how well they travel etc.
Good luck in deciding!
Joanne