[OwnedByJezebel]

I don't think that the age is an issue, but being under the 2 lb limit would worry me. I've given Sentinel to one younger than that without any problems, but they weighed about 2.5 lbs.

I have given mine both Trifexis and Sentinel, and I much prefer the Sentinel (because of the spinosad in Trifexis). I have used Sentinel on my dogs for around 15 years without any issues (and only used Trifexis for a while when there was a manufacturing problem that caused a shortage of Sentinel). If I were you, I would using only Sentinel on both of the dogs and trying to wait until the baby gains a little more weight.