My MIL tried to do the same thing....trying to pressure me to give DS "real food" so that he will get big like his brothers. DH and I had decided to delay solids with #3 because he was EBF and my older two were not due to some breast issues that arose with #1. I didn't realize before being in that position how insulting it would be especially coming from a grandparent -someone who is supposed to love your children like they would their own children! You work so hard for those 6 months and they just want to give them a taste of the "good stuff." Sorry, I could rant for hours about this. As soon as my MIL pulls into the driveway the kids are yelling to her for whatever treats she has for them in her purse so you can see she just feels that is,like a pp said, her right to spoil the kids no matter what our beliefs or rules are.
Keep up the good work!