Were humans engineered to be physically weak by our alien masters

First, NO animal to adult retaining its juvenile characteristics. That is how we differentiate between adult and child of any creature.

Second, when I was a child I thought like a child and did childish things.
No child could develop a civilization, so your basic premise is wrong and with that your hypothesis.

Problem: that's a somewhat bastardised definition of neoteny, in particular where you suggest an "adult strength". Gracile and robust forms have appeared over and over in our evolutionary history. H. sapiens and neanderthalensis. H. erectus and ergaster. There were also gracile and robust Australopithecines.

And, no. Neoteny had certain evolutionary evolutionary advantages for us, like allowing for encephalisation and cognitive development. Quite the contrary, an alien master race interested in creating a slave race probably would've (unless they themselves were quite stupid, in which case it begs the question of how they are manipulating genes) been quite wary of allowing for too much cognitive potential in their servants.

Why would they need to engineer humans when robots would be more effective for whatever they needed. Aliens are not and have not visited Earth simply because there are no reason good enough for them to travel the distances to do so.

Aliens don't exist. You might want to cut back on the TV. Get outside and play kickball.