I don't think we need to rules to be immensely complicated. Keep the same rules with 1 amendment: the player who goes first deals no damage from attacks. They can still have the advantage of first evolution and setup but guarantees both players get a turn before the game ends. I don't know the old rules because ive only played under the BW rules so these rules may very well be better then previous years but Donks are not fun for anybody(ill take one if it arises, but would prefer to play an actual game) and not dealing damage if you go first prevents donks from ever happening. I think that would be good for the game.
Unfortunately, that isn't simple. There have been many revisions to the first turn rules, and they seem to have been reversed: the rules pertaining just to the first turn of the game are now the same as they were when the game began. The differences we used to have were things like no Trainers first turn, no Supporters first turn, possibly only Supporters first turn, etc. Even I lost track years ago. At one time you may have also had to skip your opening draw as well.
That just seems a bit much for the youngest, and honestly it doesn't solve the problem. As a relevant counter-example, in
Yu-Gi-Oh you cannot attack your opponent if it is the first overall turn of the game; this is for the obvious reason that you start the game with no Monsters, Spells, or Traps in play and only a select few can be activated from hand. In Yu-Gi-Oh the primary win mechanic is depleting your opponent's "Life Points" through attacking with monsters.
The rule didn't stop
First
Turn
Kill; FTK decks just win without attacking (either through effects that do damage without attacking or alternate win conditions). Yu-Gi-Oh doesn't use set rotation but Ban Lists and... FTKs still pop up periodically because card research and development still likes to use big, splashy effects and is bad at balancing them out.
Wait! There's more; more common than FTKs decks are OTK decks...
One
Turn
Kill. The problem wasn't solved just transformed by that rule. Some of their formats were all about OTK decks.
So I maintain; the best bet is to stop implementing the card design choices that make this an issue. I want balance; that means that whatever role a Pokémon plays in a deck it does it equally as well regardless of Stage. There will be different pros and cons for each Stage, and equal doesn't necessarily mean every Stage will be equally proficient (Stage 2 Pokémon just can't function as your opening Basic Pokémon). This comes by recognizing certain things (damaging attacks, potent effects) shouldn't be available those first few turns, giving time for the fastest decks at least to set-up.
TL;DR: I don't think a rule change will solve the problem so much as change it. We've had formats in the past all about the T2 OHKO; you had a better chance of fighting back, but they were much more abundant donks. Plus the game is designed for "donks" to prevent certain abuses, like extremely low Pokémon decks (or at least balance out the benfits provided by such builds).