HI K.,
Sounds like separation anxiety to me.
Both of my children went through phases where they ONLy wanted MOM. My daughter was great with my MIL, my son on the other hand cries if its anyone but mommy, including daddy.
He's 8 months and I believe this is the reason why.
I am sure it will pass, maybe as you mentioned, leave them alone more so they develop a closer relationship.
Now, my daughter who is 20 months sees grandma and mommy becomes invisible, sometimes it actually make sme jelouse hehehe ;) In a good way, I am so glad they have that closeness for my daughter needs to have that bond with grandma and my mom lives too fat ;(
Amy