Protective dog!

Caitlyne • Bonus mom to 2💜💙 Bio mom to 1 💜
So I'm currently 28 weeks, and adopted my lab in July! He's about a year old now. 
We noticed when babysitting a family friends daughter (6 month old) that he was VERY protective over her. 
Only my mom and I could go near her without him getting iffy. 
One of her sisters (9 years old) tries to reach to her quickly and Buddy reacted in a very protective way. 
Today, I had my little brother (11 years) feel the difference in my belly between where she is and where she isn't. 
Well, buddy stood up quickly and came over and nudged my brothers hand away from me! I then tried holding my brothers hand to my belly, again, he reacted protectively! 
So there's no doubt he knows there's a baby, (my brother can hit me and stuff without getting a reaction from him)... But I don't want this to be normal. It's cute and all, but I can't have him be aggressive towards anyone that tries to touch my belly or go to see my daughter once she's here. 
We have a dog trainer that comes every week (started last week) that I'll bring it up to as well!!
But does anyone have any tricks and what not to stop the behaviour?? 
So not tell me to get rid of him. It's not an option & he's not an aggressive dog or anything! Biggest cuddler ever! He's just protective over babies & my belly!