If there is a leak of the transmission cooling coils located inside the bottom tank of your radiator, you need to get this fixed immediately. The ATF (automatic transmission fluid) getting into the coolant isn’t going to do a lot of harm, at least not a little, but coolant getting inside the transmission and mixing with the ATF will do serious harm to the transmission very quickly.
Your coolant needs to be checked in the radiator, not the overflow. If there is ATF in the coolant, you will see it when you remove the radiator cap. This must be done when the engine is cold. You need to see the ATF for yourself, even if you need someone else to locate and pull out the dipstick. The ATF must be a clear bright red. If it is brown (or black) or milky, it must be changed ASAP. If milky it must be change immediately.
If milky, or if there is ATF in the coolant, your radiator needs to be replaced ASAP, then the transmission fluid changed.
Normally I am against any kind of transmission flush, but in your case, if the ATF is milky red or milky brown, or there is any other evidence that the ATF and coolant are mixing, then I will make a conditional exception. First, no flush, but sometimes flush is a term used for a fluid exchange machine. If the shop is using a fluid exchange machine and they drop the pan and clean the filter first (this filter gets cleaned instead of replaced), then I will agree that it should be done.
BTW, you probably need an ATF change anyway, especially if the ATF is currently a clear reddish brown, but in this case, just have them drop the pan, clean it out and clean out the filter, then reinstall the pan and refill. This only changes about half the AYF, but that is enough to renew the additives and extend the life of the transmission. If the ATF is a dark brown or almost black, again do the drain and refill described above, but then in a week, have the fluid drained and refilled again, but this time, they only need to pull the drain plug, they do not need to drop the pan.