Not a pro or anywhere close. In fact these pics aren't even mine they are DH's. But as amateur as they are for studio-like work, they do show a couple of poses that worked for our 3 boys (4 yrs, 2.5 yrs, and a few mos)
I was thinking you could have the 5 yr old with the baby in the center and the other 2 on either side like this (except add in 5 yr old

)
And I'm not sure whether this would work with a 1.5 mo old, but it was great for our 3 mo old (I am just posting my old siggy with the pic in it)
Can't wait to see what you get! GL