Prune juice never ever worked for us. We always ended up using milk of mag for several days and would get DS cleaned out and back on track. At the time I was worried it was a blockage, that we'd never get things figured out and he'd be on milk of mag or miralax forever, that it'd be a vicious cycle... Last time I used milk of Mag was last summer when he went thru a picky toddler eating phase. Otherwise we've been pretty good since he was about 2.
BTW, weird toddler things. Every THING is a coconut right now. I'm a coconut, he's going to HIT me with a coconut, it was coconut-giving on Thursday, he claims he wants to eat coconut for dinner and wanted to get his grandmother a coconut for her birthday last week. Drives me batty!