Sorry you lost the other 2 keets, Guinea Hens are not always the best Mommas if they aren't in a controlled environment like the coop or a brooder pen.
She should start laying again, the laying/breeding season for Guineas is Spring thru Fall, so you are only part way thru the season with your Guineas. You will need to keep an eye on your Hen and see where she wanders off to each day... more than likely she will find a new nesting spot within a week or 2. So you may want to have a plan for collecting keets and getting her to come into a coop or pen with them once she shows up with keets again.
The box you have the keets in will need a top on it of something like chicken wire or hardware cloth (they can fly by a week and a half to 2 wks old), and a heat lamp with a regular 60watt bulb in it mounted above the box at one end would be better than the heating pad (IMO/IME they don't need a big huge red heat lamp bulb). The temp needs to be 90-95 degrees, measured on the floor of the brooder, under the glow of the lamp. And they need to have enough room in the box to get away from the heat if they need to. Then you will need to raise the lamp up a little each week to lower the temp 5 degrees (or put it on a dimmer switch so you can lower the temp that way instead) until the brooder temp is equal to the outside ambient temps, or the keets are 6 wks old, fully feathered and able to regulate their own body temp.
Make sure you are feeding them high protein game bird or turkey starter, not chick feed... they need the higher protein or they will grow and feather out slower, possibly be stunted and also have health issues down the line. They grow fast so you may want to start collecting bigger boxes, or start building a large secure brooder for them. For their best chance of survival they need to be brooder raised for 6 wks, then they will need to be penned outside where your other birds can see them for another 6 wks before you let them out to free range. The adults won't recognize them as their babies and may or may not accept them, so plenty of exposure is your best bet for being able to add them into your flock.
HTH
ETA:
I would not put them on shavings for bedding until they are at least 2 wks old... they can eat them, get impacted and die. Use rough textured paper towels, or rubberized shelf liner over paper towels for several days (no newspaper, it's too slippery), then coarse straw for a while before you put them on shavings. And try to get the large flake shavings, not the light fluffy stuff so that there's less chance of them gorging themselves on them.