Can I pull or strain a muscle in my breast by lifting my 4 year old 25 days after my breast augmentation surgery?

Thank you for your question. You definitely want to follow your own
surgeon's post-op activity instructions. This is a general guide I give
to my patients as to the recoveryWeek 1) Discomfort and tightness
level progressively decreases with each day. Swelling decreases a great
deal after one week. Most people return to work in some capacity.Week
2) Unlikely to need any narcotic support except maybe at night.
Swelling and tightness continues to improve compared to week 1Weeks
3-6) May need tylenol or ibuprofen for intermittent discomfort.
Swelling completely resolved. Tightness may continue as the implants
over an additional few months into their final position. As far as activity, here is a safe guide:1) No heavy lifting or strenuous activity for 6 weeks (a gallon of milk as a guide).2) Resume walking at a leisurely pace right after surgery (e.g. 2 mph)3) At 2 weeks, you can walk 2 miles at 2mph4) At 3 weeks, you can walk 3 miles at 3mph5) At 4 weeks, you can walk 4 miles at 4mph6) At 5 weeks, you can jog 5 miles at 5mph7)
At 6 weeks, you can resume all activities, but listen to your body and
use discomfort or tightness as a guide so you don't over do it. So
as far as the little ones, I would recommend having help for basic
needs like lifting into the car seat. You can cuddle with them while
you are sitting down, but you should avoid the tendency to multitask
with one child in your arms while you are busy doing other tasks.If your pain from your recent activity doesn't subside or gets worse, then I would visit your plastic surgeon.  Otherwise, continue to be cautious and not overdo it.  I wouldn't worry about capsular contracture from this.

You certainly can aggravate your pectoralis muscle by lifting your child. The muscle is already irritated due to the breast augmentation. You should rest and medicate the muscle if possible. A reevaluation by your plastic surgeon  is appropriate. 

It is certainly possible that you strained a muscle while picking up your child.  This should not lead to capsular contracture unless it caused some bleeding, which would not be expected 25 days following surgery.   You should talk to your surgeon about your concerns.