Is my 4yo son's time with his grandmother a right or a privilege? LONG

LONG- Tl;dr at the endSo my husband and I are working on being consistent with discipline with our 4yo son. I'll admit that I don't feel like I am very good at parenting a preschooler, but I am trying my best. We also have 2 younger daughters (2.5y & 8m), so our energy is minimal. Most of the time I feel like I am just firefighting, but I am trying to get to the root causes of some of my son's more challenging behaviors.In general, he is a kind and courteous kid, but sometimes he is very broody and physical in his responses when things don't go his way. Lately, I've been working hard to affirm the emotions he's feeling (e.g., frustration, anger, etc) and helping give him tools to work through them. We brainstormed together and came up with blowing air out of his mouth, pretending like he's a dragon using fire to burn his frustration up. It has helped him a bit, but we're still working on it.As far as discipline goes, we typically revoke privileges and find that to be effective with him. Both my husband and I were spanked growing up and do not feel comfortable with that for our children. We have used time outs (i.e., removing him from our presence and his toys) and/or taking a toy away for a set amount of time, usually the rest of the day, but sometimes a shorter length of time like until after lunch or something like that.Here's where the grandmother situation comes in-my mom comes out one day a week to help with the kids (I'm a SAHM). Sometimes I have appointments or errands to run during these visits, so I am out of the house. Other times, I work on projects around the house. When I am home during these visits, my son has been horrible to me lately. Like just mean and nasty, out of character and embarrassing. I know it's mostly because he's competing for attention and wants her 100% to himself, and I think this behavior, to a degree, is normal kid behavior, not good, but relatively normal.On her last visit, I told him in front of her, "If you continue to act this way, Gigi may go home early today." In my head, time with his grandmother is a privilege that can be revoked temporarily (similarly to time out away from my husband or me) and would obviously be restored at a defined point.My mom, who is quite sensitive and has some emotional baggage, was extremely hurt by this statement and felt like I was positioning her in a way that forced her to abandon him that would damage their relationship. Obviously, this is not what I meant to do, and I think ultimately she and I view time with her differently, i.e., a privilege vs. a right.So what do I do here? In my mind, I feel like this discipline is consistent with what we do in our family, but it hurt (and will continue to hurt) my mom's feelings if I don't find something else that works. Obviously, I don't want to hurt her feelings, but I also don't want mine hurt when my son is mean to me. Help?--------Tl;dr-My 4yo son is mean to me when my mom, his grandmother, is around. My husband and I typically use time out or take away toys to discipline him, and I view time with her as something that can be taken away for a defined time when his behavior is repeatedly disrespectful. I said this in front of her, and she was extremely hurt, saying that it forced her to abandon him and damage their relationship. Who's right here? Is time with his grandmother a right or a privilege? via /r/Parenting http://bit.ly/2WzhhTo