I'm not sure whether any day(s) would be better or not, but I'd check to see if they're open and offering the option on Mondays. Typically Monday is a day that museums close down. (We found this out the hard way one year!) Though, I don't know specifically about the one in LA. It's been years since I've been there, but went quite a few times when I was younger. I don't remember any really truly busy time, even when there were a bunch of school programs.
That being said, if I had to make a "guess", I'd go with Monday, if they're open. My experience with school/daycare trips is that they don't tend to go on Mondays as frequently as other days.
Have a great time. I'd really appreciate if you could post your experience here when you get back! I found out about it and was thinking about going while we're there the end of March/first week in April, but found out it's coming here to the Minnesota Science Museum, so my family wants to wait and go here.
