The goal of signing with your baby is to communicate better with your child, and when your child is learning to sign they will often sign back in ways that are a bit different than how you sign. So while I think it is important for you to sign correctly and the same way every time, it is fine that your child may sign back in a more relaxed or shorthand manner.
For example, the sign for dog is to slap your leg and then snap your fingers (as though you were calling a dog). When our child first signed dog, she just hit her leg a lot — and that was great. She also generalized this sign at first to cover all sorts of animals, and that is common as well to. We would just say to her, yes, that is a (whatever the animal really was, a squirrel, bird, etc), and make the correct sign for that animal so she would begin to pick up on the different signs for different animals.
I know that often you may find that your hands are occupied, which makes signing with your child a bit more challenging. So in these cases, just sign with one hand as best as you can. For example, the baby sign for cat is two hands tracing whiskers on your face. But if you only have one hand free and you see a cat, you can just use one hand to trace the whiskers as you say “cat”. This is actually the ASL sign for cat, but when signing with babies, I find it’s more effective and iconic to use both hands.