Should I tell??

So I need to know if I should tell my ExHusband that I'm pregnant by my boyfriend. My Ex Husband and I have 2 kids together and he is starting to come back into the kids lifes again so im not sure if i should tell him or let him find out on his own. My Boyfriend thinks we should tell him out of respect for our kids but I am soo scared to tell the ex husband. Also me and my ex just got back on kinda good terms but he still thinks we will get back together. Any Advise ladies???