Is it safe for breastfeeding mom to use baby sunblock?

Absolutely. Not only is it safe, it is a great idea. We all need to protect our skin more. Also, since you are breastfeeding and planning to spend time out in the sun, increase your fluid intake a lot on those out-in-the-sun days so that your milk supply doesn't drop off from not enough hydration!
Sunblock. Yes, you should be able to use baby sunblock without any difficulty.