you should not skip lunch, never! 50kgs seems a little underweight so first of all gain some weight then your arms and wrists will feel healthy too. Secondly don't worry about thin wrists, i too have thin wrists but as far as they are strong and have good strength, you don't need to work on them at all. If you still insist, then try doing light wrist exercises but don't overdo it as there are no muscles in wrists, you cant really make them thick. And overworking the tendons can make problems. Just eat healthy and balanced diet, do light exercises and you are good to go.
yes ensure helps but if you can get your hands on to natural diet, why go for such things? use it as an add-on but not replacement to your meal no matter what's written on the label 
edit: a funny but good example, a desi murgha is so thin yet it is so much strong, but a shaver murgha cannot even carry its own weight. You can easily see them crawling on their thighs... Try to be a desi murgha instead of bulky powerless shaver 
Also eat more meat in your diet, proteins help building up your body
