I put 23 chicks straight from my incubator into my brooder built into the coop this past Monday. I’ve already had temperatures below freezing and in the upper 60’s. You mentioned transitioning outside, I’ll come back to temperature swings, that can be an issue outside but generally not much of an issue if you brood inside in a controlled temperature. I use heat lamps but am not insulted by your preference. We all have our own preferences and reasons. The other methods can work great if they are set up right. The basic principles are the same.
The heating pad cave or one of those other heaters will work great inside. I don’t know how many chicks you are getting, you have to look at capacity. I don’t know if the two devices mention capacity. With the heating pad cave you can get different sizes of heating pads so capacity can vary.
You do not have to cover those pads with any bedding, though insulation will help trap the heat. The chicks will spend a lot of time on top and they poop a lot, you probably want something to protect that pad from the poop.
Any time you use electricity you have issues and risks with possible shorts or such so pay attention how you route that electricity. That doesn’t matter if you use an EcoGlow, heating pad, or heat lamp. The wiring inside an old pad can become brittle and break, which can cause a short, but as long as the pad is in decent shape they aren’t much of a fire danger. They are a pretty safe option whether in a kiddie pool or anywhere else. There are people on here with a lot of experience with the heating pad caves. They can tell you of the things to watch for and how to set them up. Getting the temperature right and making sure the chicks don’t get trapped if you use a lining or cover are a couple that come to mind.
The basic idea with any of these methods is to provide one area warm enough in the coolest of temperatures and another area cool enough in the warmest temperatures so the chicks can go where they wish. The chick are extremely good at self-regulating their temperatures if they have a choice. In a climate controlled area where the temperature stays pretty constant, this isn’t really hard.
Outside with big temperature swings this can be a bit more challenging. I have no idea what your temperatures will be in April or May when you will want to put them outside. You may not either. I can’t tell you when they can go out without supplemental heat. For most of us that is going to be around 5 weeks of age. That will depend not only on your outside temperatures but on how well they are feathered out, if you have been acclimating them to cooler temperatures, how well their coop is ventilated but has good breeze protection where they are. Good ventilation up high helps but you do not want rain or a breeze hitting them.
I don’t know if I’ve answered all your questions. Indoors in a kiddie pool the heating pad or those devices should work great. The heating pad can go outside in any weather, the two devices I’m not sure what their limitations are outside in temperature swings.
