Mom of three trying to build muscle!

Hello everyone! I initially weighed close to 200 after my youngest was born. As I’ve surpassed my goal weight, I set a goal to get down to 130. I’m close to that, but I’m starting to think more about body composition now. I’d like to build muscle in my arms, and my abs. I’ve been doing lateral raises and biceps extensions for my arms. I’m trying to work the backs of my arms especially. I’m just not sure what’s best for my abs. I’d like to work my lower abs especially. Any ideas on good ab exercises or maybe something I could add to my arm exercises would be very appreciated!