Inside your house in the low 60's. That kiddy pool is probably a decent size. They are Silkies that can't fly so an open top is fine. Many others could already be flying out.
My suggestion is to set that heat lamp up so it only warms part of the kiddy pool. For safety reasons do out use that clamp that comes with it. Do not use string or plastic that can burn or melt. Use wire to firmly keep it in place. You do not want that lamp falling and starting a fire. Fix it so that cannot happen.
To me the ideal is for part of that brooder to be warm enough while the far part cools down to ambient temperature. That way the chicks can be where they want to be. That takes all the stress and worry off of you trying to get them to the right temperature. Let them do the work. They know when they are comfortable better than you do. How do you know that you have an area warm enough? Watch them, they will tell you.
I put chicks in my brooder in the coop straight from the incubator or post office, even when the outside temperature is below freezing. One end is toasty warm even when the far end might have ice in it. They are smart enough to stay where it is warm even when that young. When they get older they spend time in the far end at whatever the ambient temperature is. That way they acclimate themselves. I let them do the work. That way when I'm ready to take the heat away they are ready.
Yours are indoors where the lower 60's is the best you can do that way. If you can, I suggest taking them outside in colder temperatures and let them play. I like having them out where they can peck at the ground anyway, I think that helps strengthen their immune system and gets grit in their systems. But this serves two other purposes. It helps them acclimate to even colder temperatures. It also helps you see how well they can handle colder temperatures. Seeing it for yourself should be much more valuable to you than what a stranger over the internet like me tells you.
Good luck!
