It depends on how deaf the child is. If the child has some hearing, then learning BSL will help them better communicate with Deaf people. If the child is profoundly deaf, then BSL will not be of much use to them.
It takes an average of 2-3 years to learn British Sign Language (BSL) fully. There are different ways of learning BSL – some people prefer classes, while others prefer online resources. The most important thing is to find a method that works for you and stick with it! Make sure you practice as often as possible – the more you use BSL, the better you’ll become at it.