Best Practices for setting up a exchange server

I have a root DC and two children,the purpose of these children are to service different set of clients. I also have a two seperate email servers which service each of these clients. Both emails servers are external and seperate DCs. I would like to know what is the best pratice in setting up the email servers. We have a barracuda that can serve as a forwarder in this environment, is it ok to make the email servers internal and join them to the domain of each child and let the barracuda forward the email to them?