How to bring up a concern with my mother

So I think this would be the right forum to post in for my question. Today my mother was watching my 2 children (1 yr old son and 3 yr old daughter). She has watched them MANY times and I have never felt any need to be concerned. I am a stay at home mom but will leave the kids at my house with my mom to run errands sometimes to make it easier on me (I'm 7 months pregnant). Well, today my husband was working from home and happened to notice that our 1 yr old son was in the kitchen unsupervised for a period of time. Both children were originally in my daughters bedroom playing with my mother. She seemed to not notice that my son disappeared into the kitchen. We live in a rancher so there is only one level. Even when I'm watching the kids my son tends to wander BUT I always go check it out. When my husband went to check on the situation to see why our son was by himself in the kitchen my mother was doing work on her phone. She is a realtor and I know she does a ton of work on her phone. She tends to be a workaholic and not able to tell people they have to wait until later in the day to get a response from her if shes busy. My concern is that I don't know if this is a common occurence when I leave the kids with her. When my mom and I are together she tends to be on her phone for work a lot but I don't mind because I'm there to watch my kids. I just don't know how to bring it up nicely with my mom. We are expecting our third child in March and I was really looking forward to having her help me out after my husband goes back to work BUT I'm just worried now. Anyone have any advice??