There is a difference with being a helicopter parent who is on top of the children all the time at school, play, etc.
What you describe is not helicopter parent... it is your basic concern for the health and safety of your children.
They are 4 and 6, they need constant supervision.
As for the seat belt issue.. it is a law here that everyone is buckled in or you get ticketed. I don't consider it a "treat" to not wear a seatbelt or sit in the front... that is asking for trouble. God forbid there is an accident.
Sunscreen is vital at the beach and should be for everyone. I am of Native American heritage, never burn, tan well, etc but I ALWAYS wear sunscreen. Basic health safety.
I think you have answered your own question. You want your children to have a great experience but your know the parenting style of your SIL and it is not like yours.
Like I said, it is not being a helicopter to watch children when at a beach or pool... it is something parents should already know to be diligent about due to possibile drowning, getting a little too far away, etc.
If my child were 4 or 6 and I were faced with this type of parenting style for her to go on a trip, I would politely decline. I am FAR FAR away from helicopter mom, I encourage independence, but I would not knowingly let her go on a trip with someone who I knew was lax with parenting. if they don't keep their eyes on their own children well, they won't with yours either.
If you do decide to let them go, make sure you talk to the BIL AND SIL and they know your expectations.
Never compromise on health or safety