Hiya Sandy!
Welcome to planDisney!
Sci-Fi Dine-In Theater Restaurant is such a unique dining experience and I can totally understand why you three want a chance to experience it. I love how you want to sacrifice your chance to enjoy it to let your little ones eat alone but unfortunately, it will not be possible. Your kids are too young to eat alone in a Table Service Restaurant. The reservation must be made by a person who is aged 18 and older and while the person who enjoys the reservation can be different, the
Walt Disney World Resort Property Rules state that guests need to be 14 or older to enter and enjoy the Parks without supervision. It also states "Children should be supervised." Therefore, you will have to accompany your little ones to their meals.
I however will not leave you hanging my friend. My suggestion is for you to try and book a reservation for a party of 4 and try to modify the headcount after. Sometimes reservations with even numbers do better. Most of the cars used for dining in this restaurant comfortably fit four in pairs. If only a party of 2 is available then just grab what you can get and keep an eye on modifying it to a party of 3. I have found that the closer you get to the date you wish to make a dining reservation, the more likely one will pop up. As people change their plans and cancel, you can use that to your advantage. I would continue to even check the day of as reservations can be
canceled with no penalty up to 2 hours before. If you find that on the day of your reservation, you find no way to modify your reservation to more than 2 people then I would head to the restaurant at least 20 minutes earlier and speak to the Cast Member at the Front Desk. Kindly ask if it is possible to increase your reservation by one more so that you can all eat together and be willing to wait a bit. It is not guaranteed but I was able to do this on a trip a few years ago. Since it is not guaranteed I would continue to make sure to keep trying to secure the right reservation headcount. I do think you will find a time if you're persistent.
I hope this helps and I wish you all a wonderful time at Disney's Hollywood Studios!
Happy Planning!
-Nimmy