It's hard to say without knowing exactly how much changes in the dendritic properties of PC cells you expect between P10 and P15. To be confident, I would suggest neuronal modelling, but that will probably take too much time unless you already know someone who is very competitive at multicompartment modelling.

I'm somewhat tempted to suggest something like the following: fill you PC cell with Alexa, so you can visualize it's dendrites, then, at a proximal section of dendrite, puff a high sucrose solution from another pipette. This *should* cause release of synaptic vesicles. Then somehow quantify the magnitude of the release event. If you do this on animals at P10 and P15, you should be able to keep the distance from the soma constant. The sphere of influence of the sucrose should be constant. Hence the only significant change should be the number of active synapses within that sphere of sucrose.

It's not perfect, as if there are large changes in the membrane properties you could get different space-clamp properties, and you'd need a microscope with epiflourescence (unless you're DIC is good enough to visualize dendrites with confidence) and if the nature of the neuropil changes significantly, perhaps the sucrose could get further (though I doubt the last one). Still, it's as good a solution as I can think of at 9 in the morning.