Are your parents obligated to your stepchildren?

Nathalie 🌹 • "Life isn't about waiting for the storm to pass...It's about learning to dance in the rain." 💃🏻
Hi Everyone,
I have a 20 mo old son and a 6 yo stepdaughter. Recently, my husband has been voicing his objections about my parent wanting to spend time with my son individually, as opposed to both children together as a unit. My parents have included my stepdaughter on outings, but I understand them wanting to spend to spend time with my son alone.. they are brand new grandparents, he is their first. I don't want to force that, or change up their routine whenever she comes to visit. I feel they deserve to enjoy their grandson whenever they want. 
How can I make this transition easier for our family?