Ah it's a MIL, who arrives at yours at 1pm, asks her DD to come to yours after school and she stays and wants to have tea cooked at yours with her 10yo DD, but you have a baby who eats different food for their tea, and you & DP eat after baby is in bed.
You don't want to offend MIL but it's ok to say what you want and don't want to do. My DD(age9) has a fab saying "No, I don't want to..(today)", it works...
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Sounds like MIL is wanting to stay til DP is home and is using it as 'family get together' time for her DD too, and for her to see her DGD baby & you for a few hours as well. Which would be fine if she said that or you asked and wanted that 6 hour visit.
But it doesn't sound that every time you really want such a long visit. So why not say- when you aren't prepared to have a 6 hour visit-
"Come at 5:30pm after you & DD have had tea to see DGD and DP too...."
".If you're coming at 1pm to pop in to see DGD that's fab, but not one of your long seshes as I like to have a a few quiet hours cuddling in peace with DD before DP gets home and we start her lovely bedtime routine. ."
If then over stays, & suggests she & her DD stay for tea & she pops out to get it, it's fine to reply "Not today MIL, it's been lovely seeing you and DD but we have dinner plans already, & will be eating later at 7:30pm and I need a bit of quiet time before then. Off you pop now, so you can get home in time for (your) DD's tea"