I agree w/pp's...if you feel like you don't want to be away from your little one right yet, you can do a couple of things. First you could just start the reading part of the training, and have it done by the time your baby is ready for mama to leave them alone for a day or so at a time...you could also do as pp suggested and check out ALACE...do the reading for them, host a training (so it's free, and that way you don't have to pay to travel or for lodging!), and then finish the births when it's right for your family.
I have a 3.75 yo and have been doing births for...oh, about a year and a half. It was a lot harder when he wasn't fully night weaned. The first couple of nights I was gone he really gave dh a hard time, but after that, dh would just snuggle him and when he realized that I was "at work" he would just cuddle in to dh's shoulder and go back to sleep...I think it was harder on me knowing that he'd miss me, than it was on him ('cause really he had his daddy and likes him better at night anyway...he'd nurse, and then roll over and cuddle dh!).
I notice that my kids DO really notice when I'm gone if I have more than a couple of births a month. They'll start to act up when I'm newly home, I think to let me know that they're not so keen on my being away that much. I completely understand, and truely wish I didn't have to be away from them at all. Unfortunately, we need the money right now...and my other job (being a teacher) wouldn't provide NEARLY the flexibility as being a doula does. Course, a teacher gets paid a lot more, but is gone a LOT more, as well. ANYway, sorry for the tangent. Point being that if you know your kids, you'll know what's best for your family. I can be at two births a month and my kids see the time that I'm away (if it entails daytime absence) as a FUN visit with their grandma or best friends. However, they're bothered by more time than that.
Good luck in your choice.