Get Started!
Speaking as one who has two kids (daughter 3 1/2, son 11 months) and two projects (RV-10 and RV-3B), kids will DEFINITELY have an impact on your build time! That being said, I believe that if you have the money to simply purchase the entirety of an RV-10 kit, then "saving money over time by buying now" might not be your real concern. It may be that you're questioning whether or not you'll have time to build with a kid(s) in the picture, and wanting to "lock in" your build by having a kit in the garage. My suggestion is to have a long, honest talk about it with your wife. She will likely have a difficult cross to bear if and when you start on your project. I'm not saying you won't be around at all (that would quickly destroy your relationship with wifey and kiddie(s)), I'm saying that when you're busy working, your wife will be busy doing dishes, laundry, cleaning the kitchen, picking up toys, laundry and probably more laundry. You both don't yet know what's in store for you, regarding time management, but you can at least speculate on your specific roles and make sure that she's on board for her expected sacrifices, and you yours... You too will have times, when you watch the wife and kid(s) pull away to spend a beautiful day at the zoo, waterpark, whatever, while you get to debur/dimple thousands of holes, research proper edge distances, torques, etc. You friends will question your loyalty, since they don't see you as often. Etc, etc. Try to know what you're in for, then make the leap or not.
I knew a guy that built an RV-10 pretty quickly, spending all his time in the shop while his wife cared for their baby "TWINS!" When he finally had it built and wanted to fly places, the wife and kids had little desire to go with, as they were into other things and used to daddy being "busy." In the end, he sold the plane about a year after it was finished, to buy a boat. So, my $0.02 is to talk about it, make sure you're both okay with the sacrifice vs. payoff, and if so, then pull the darned trigger!