OP, congrats on your little family and it’s soon to grow by one!
I noticed a couple of things in your posts:
You have a “grandparents expectations” problem. You siblings have set an expectation regarding their children and their parents, and you parents want to apply that to you. It’s understandable, but it doesn’t mean you have to change anything. They are wrong.
Be very very careful about setting any kind of formal or regular alone time with them. Based on your posts they will come to feel it is “their time” with you son and you will be on a custody arrangement with them. Any changes to the “schedule” will have them crying, demanding their time. It will get ugly. Your parents have a hard time separating different people and their relationships with those different people. Be kind to your parents by not letting hem set an expectation of alone time with your child on a schedule.
Keep the “alone time” with any of your children at the convenience of your family. Healthy grandparents with evolved lives don’t bees to demand alone time with someone else’s child. It’s highly inappropriate. They do not need your child to make them happy. They need to find happiness in their own lives and at the stage they are currently living.
Don’t listen to anyone who tells you that your child “needs” grandparents. It’s a self serving manipulation tactic. Your child needs a loving family. That family is made up of well balanced people with your child’s best interest at heart. It if you and your partner, siblings. If it includes grandparents that you, as the mom, enjoy being around great. If not, no big deal. Your child’s family is also made up of others who truly care about your child. Teachers, friends, parents of their friends, mentors. Just because your parents share DNA with your child does not make them qualified for alone time to qualified to influence your child.
Please, do not make any changes between now and when the new baby comes. This will be a critical time for your child. Soak up as much family time you can as a family of 3. Once the new baby comes hunker down as a new family of 4. The absolute worst thing your can do to your 3 year old is change their routine while your family changes so drastically. Do not send your child away. That would be awful for your child. It’s a hard transition, welcoming a sibling, don’t make it harder by taking tour son away from his mom and dad.
Above all, don’t ever do anything you are not comfortable with regarding your children. Listen to that feeling that told your your parents request isn’t right. Their demand, their expectations, their wording is all very wrong and a warning sign.
You are a great mom. Don’t let anyone in your life or on this forum make you believe any different.
Keep doing what you’re doing. Hugs!