Stray Cat And Kittens

Last night my Step mom, brother, father and I found a little of kittens in my dad's boat in the back yard. Their eyes aren't open yet, but the umbilical cord is gone so we're guessing they're about a week old. There were four of them. We found them about 7pm last night.
We were getting ready to work on the boat to replace some lights and the wiring is right there where they were, so we removed them and brought them inside. Bought some formula and I got up every two hours to feed them, burp them, make them go pee, ect.
This morning I did the same, but we wanted to return them back to the place we picked them up so I proceeded to head back to the boat to put them back. As I lifted the seat up to put them back, momma was staring straight at me and bolted away as quick as she could. She had another kitten in there so I know she was moving them.
This was about two hours ago, I have since put all the kittens back and put food and water there for her near where the kittens are. I'm inside, the boat is out back and the door is shut. I've been sat here eating lunch and keeping a close eye on the boat to make sure she's coming back. I never meant to scare her.
My concern is how long should I wait for her to come back? I don't want to scare her again but I don't want to leave the kittens as it's a very hot day, in a boat, under a seat, a boat cover and under the hot sun. At least for too long.
I'm certain she will come back, I did leave food and water. She seems semi-feral as she didn't hiss, she didn't growl. She just bolted and left her other kitten behind. There are five total that I know about, and since she's moving them there could be more.
Advice in this situation would be swell. I don't want to go out there, check and scare her... Again. :/

I now next to nothing about how new moms react, so I can't help there. But honestly, if you can foster them, I would take them all in now. If mom is too frightened to come back, the kittens will not do well out there.

Thank you for your quick response, should I post a new thread there or get this one moved? I want to make sure they all survive, but I know it's very important at this age for the kittens to be with their mom. I would love for her to come back and take care of them, but if not I will.

Still no sign of momma, three hours. It's hot outside. :/ I'm concerned the kittens are sweltering in there.

she may be startled=best bet is to go back and get the kits and feed them-they are too young to go this long without food and help with peeing etc. Maybe set up yourself out there by the boat-feed them there if it's in the shade or near there-she should hear them crying. Kittens are noisy. Try to entice her inside with scraps of good food-then keep them together.

What happens if she's there and I scare her off again or can't coax her inside? I know it's very important that they have their mom at this point in their life. :/

just sit quietly. don't talk. Don't look at her. She will come back. Withhold her food for a few hours-she will come back. feed her around 7ish...do you have a garage you can entice her into? Or other small building? You can have a cat carrior with food at the very back-then close the door when she goes in-and move her into your bedroom or bathroom. set up a banana box with blankets-she will be comfy in there with her babies..she will be skiddish at first-just don't talk for the first few days=let her get used to your scent-then feed her treats and don't look at her-remember face to face is combat to cats=keep your body turned sideways when around her-this is cat body language that you are not a threat.

for now grab those kits and feed em.

My step mom has told me to leave them. She'll come back. My heart aches, it's their house and I'm here visiting from another country so I'm staying at theirs.
I'm still keeping an eye out.. I did walk to the side of the boat and listen, no noises but I'm not sure they made them before now. We never heard them. I kinda gave a glance around our alley and down the street I saw her dart to. Just to see if I could spot her, nothing.

Update! We brought all five in and fed, peed and pooped some. Put them in a basket on our deck. I went through the neighborhood and found momma, put her with the kittens and now she's picked one up and left with it. Will she be back to get the others? I hope so. Momma let me pick her up, growled but didn't lash out.

Momma hasn't been back yet, called a vet she told me to leave them out with food and water and if she doesn't get them by sundown I have to take care of them again (which I don't mind but you guys know it's lots of work). She said if we could get momma again and her other baby confined, she'll raise them. It's just... Going to be an interesting adventure.

Update, again.
I hunted momma down again and brought her back to her babies, they nursed for a bit then she grabbed one and ran off. So three left. They're outside in a basket, exactly where we left it when she nursed. Hoping she comes again to get them! **Fingers crossed!**

She's taken another, I repeat, there are only two left. Come on momma! Please keep your family together!

Is there a spot you can secure them all together so she can't leave? Like a garage?

Not really, my parents garage is filthy. They have a hole they haven't repaired so birds get in and it needs to be cleaned desperately.
Momma came back and got one more kitten, two left. I brought them in for the night and have been taking care of them. Will be trying to put them back outside later to get her to take the remaining two. Fingers crossed!