I think there's a misconception in what goes into answering this question. There are two separate processes that address distinct issues:
1. Public Health/Government Decision
At this level, the decision is NOT about whether it is safe for kids per se. Obviously, if it was particularly bad for kids, that would be a consideration, but the primary concern is the effect on the population generally in the area within the jurisdiction of the governmental unit. That's why things like the size of the gathering, the number of people from out-of-town, whether asymptomatic carriers can spread the disease, the means by which they can spread it, all factor into the equation.
The government lets people go about their lives freely in the flu, even though it can and does harm and kill people, because they believe the overall spread can be contained through vaccines/existing immunity and the public health system can handle the load. For some illnesses, they require kids to be vaccinated to attend school even though the illness isn't really that bad for most kids, in part because they think it's bad for enough kids and the costs of the vaccine are low enough to make it worth it to prevent the spread. The gov't also lets people do all sorts of risky activities that have a higher risk or incidence of death (e.g., driving) as long as they meet certain safety guidelines (passing a driving test to receive a license) because they believe that is enough to contain the overall pressure on societal resources, even though they know that driving will result in more deaths than if everyone walked.
So, this why no one is discussing whether kids get sick, although they may be discussing whether kids can transmit the virus. They are instead focusing on things relevant to transmission rates, such as masks, group sizes, degree and amount of contact, testing, isolation etc. They want it to be phased in slowly, and with certain precautions like masks, to reduce jump-starting the rate of transmission and thereby keep it from overwhelming societal resources. Some people believe herd immunity would be a better way to deal with this issue, but whether government chooses not to follow that is not necessarily because they are trying to save literally everyone in a bubble, but because they have doubts about whether it will work sufficiently to stem the tide or whether it will backfire and accelerate the spread. Sweden's experiment is inconclusive and their population demographics may not be a good match for us.
2. Individual risk
Ending or relaxing the stay-at-home order doesn't say much about whether it's safe for you individually. Once the government and public health authorities move out, that is really just a hand-off of the risk to private actors. That's where groups like Cal South, insurance companies, permit-issuing authorities concerned about facility liability, and clubs come into play. They are considering the question of whether kids can get sick, how badly, and under what level of contact because at that stage the government has withdrawn and the private actors are allowed to decide for themselves what is an acceptable level of risk. The groups may refuse to restart or adopt protocols that are often stricter than what is required by the public health authorities because legal liability is a function of the reasonable standard of care. That standard is about what is reasonable to avoid harm to an individual, whereas the public health standard is about what is necessary to avoid harm to the community more generally. The former is often high than the latter, although there are ways to reduce what is the reasonable standard of care such as broad governmental grants of immunity that effectively shift the burden to the participant or parent to make the decision. Waivers are an attempt to shift that burden privately by contract, although courts will scrutinize them to make sure that they were granted voluntarily and with full information, which is usually where they fail.
Bottom line - we're still at Stage 1, where the effect on kids is less relevant than whether they can be asymptomatic carriers. The effect on kids comes into play in Stage 2, which is why you could see governments open up, while clubs still have difficulty fully resuming full play. Only at Stage 2 is it relevant that a parent is willing to take the risk for their kid, since they may be willing to sign the most ironclad waiver available (although insurance companies and facility owners have to factor in the risk that the waiver won't hold up in court even if you think now that you will sign away all of your rights forever). Assumption of risk is irrelevant in Stage 1.
