This happened to me too as I played yesterday. A toddler couldn't interact with his grandparent at all. I thought it was weird, but didn't think much of it until I saw this thread (I can't remember whether I have encountered the same problem before).
I dealt with it so that in manage households I transfered the toddler to the grandparent household. That gives them immediately a relationship boost. Then in play mode I clicked the toddler with the grandparent and chose "adopt as caregiver" and with the grandparent I clicked the toddler and chose "adopt as care dependent" (and did the same with the other grandparent as well), so that they were able to take care of the toddler easily. The toddler spent a couple of days visiting his grandparents and it was lovely. I like them visiting a bit longer 🙂
Anyway it is strange that, after I had transferred the toddler back to his parents I added a second toddler to the family in CAS and invited the grandmother over, and this new toddler can interact with her grandmother right away and has the same interaction options for her mother and the grandmother, though she hasn't actually even met her grandmother yet. I wonder why she can interact with her grandparent right away, but her brother couldn't.
// I transferred the second toddler to the grandparent household and it turned out she already had her grandmother and grandfather set as caregivers, though I hadn't done it! I assume it is because her brother had grandparents as caregivers, so the game somehow copied this setting to her as well.
