Toddlers and Boobs
Toddlers will touch your boobs mostly just because they don't know they're not supposed to. If you breastfed they may not understand that the girls don't belong to them anymore. If you're still breastfeeding that makes their relationship with your boobs even more complicated, LOL. Even if you didn't breastfeed, kids get curious about body parts at this age. Especially if you're potty training because private parts are a frequent topic you discuss. My brother was never breastfed but I remember my mom having to sit him down and talk to him about why it wasn't okay to touch her there. He just thought it was funny because she'd react when he did.
I think the best thing to do is to just pull their hand away and explain to them that they're private parts. No one else is supposed to touch them (or, if they're still breastfeeding, you can only touch them if we're nursing). You'll probably have to remind them several times before they get it. Just try not to make a big reaction because they might think it's a fun game to play with you.