Lead based paint laws became effective in the US after 1976. If your house was built after 1976, then you needn't worry about lead based paint testing, and can sign the waiver without a waiver.
If your house was built prior to 1976, there are a couple of danger signs to look for, especially the one you mentioned, flaking paint. Lead based paint flakes differently than latex paint. It flakes in regularly patterned, symetrical squares. Provided there isn't any flaking paint, and you don't allow your kids to lick or ingest any paint, you should be ok UNTIL SUCH TIME as you remodel/repair. If you remodel and break the seal on the paint so that it can disperse into the air, or chip, then you run a potential risk. So look at the house carefully, and try to imagine what you might do to it down the road, and how easy it would be to tent those areas off, etc, when you are working on them.
There are two remedies for lead based paint issues: the first is to seal it by painting over everything; the second is to tent, strip, and then dispose as if it were hazardous waste. The first is something you can do yourselves and probably would anyway, so then the expense of the test is probably not worth it. The second solution is so expensive many sellers have second thoughts about selling and pricing when they are asked to do this.
First, check here: EPA Map of Radon Zones
Radon is a serious issue in newly built, air tight homes. In older homes that "breath" the gas is less likely to exist in concentrations that appear to be consistent with health problems.
Radon is less likely to be found (although it CAN be) in a home
built on a slab as opposed to one on a basement.
Radon tests performed when the sellers are still in the home can be inaccurate, since the test calls for the house to be shut up completely for a period of three days. It's an unusual seller who will leave their home shut up like a tomb in the interests of an accurate test that may jinx their sale.
D-i-y radon tests can be purchased at many hardware stores for as little as $40. Most inspectors will charge you a lot more, since it requires them coming out to the house twice.
Radon abatement isn't difficult and can be inexpensive. Very often, it is as simple as installing a vent and a fan in the basement. Some contractors might recommend painting as well as sealing cracks and covering all exposed earth.
Alternatively, you can ask your realtor to include the radon contingency, and then purchase and place and retrieve the test yourself. Before you do this, make sure that your contract doesn't call for this test to be done by someone who is licensed to do so, or your results, even if they are bad, will be useless to you.
The newest radon issue is radon in water. Local municipalities are not required to test for radon in wells or public water systems. In fact, I am not sure the EPA has even established an acceptable radon-in-water level. I do know that getting radon-in-water tested is expensive because there are very few people approved to do it. As far as I know, the only abatement method is a carbon filter, which is expensive and gives rise to issues of disposing of filters that are radioactive. This is one test I would absolutely do, given the expense and difficulty of repair. Do not allow them to tell you that it's too late, or that it is not covered in the original contract contingencies. There is case law that says "radon inspection" is just that, and is not limited to air testing.
If you include radon contingencies in your contract, do not forget to include details making sure you are allowed to re-test after abatement, hopefully, at the seller's expense.
Hope this helps.
(Edited to add: Neither radon nor lead based paint should exacerbate existing allergies. Radon is associated primarily with lung cancer and lead with brain damage from concentration in the blood.)