I personally waited until my kids were at least 18 months old because according to the CDC if they catch it earlier they are more likely to have shingles at a younger age and also potentially catch chicken pox again. It's all in the Pink Book chapter on varicella on their website.
I wouldn't be comfortable purposely exposing a 6 week old, which in essence is what would be happening by exposing your 3.5 yr old. Breast milk is not 100%. I had chicken pox as a kid, then I received the vaccine when I was in the military, then I had shingles just 2 years ago...dd is still nursing 5+ times a day and yet she's the one that caught the pox, not my 5 yr old. I didn't even really purposely expose dd except to have her in the same room as the infected child. My 5 yr old was doing the suckers and all the other stuff and never got it. And he was only nursed for the first 4 months...and yet he's always been my healthiest child, go figure.