We started our club last week and had 25 kids show up! Not too bad for a first meeting. Question came up that I hadn’t thought about but being as these kids haven’t played much, if any, chess but they came up with it. We were in free play time at the end and a kid asked me this… What if a player has made a couple moves before one or the other realizes one of the players is in check and has been, what do we do? We haven’t started notating moves yet, we will start teaching that tomorrow, so what does one do in a situation like this?
Are you (attempting to) follow US Chess rules? Except for keeping score, that is. If so, go back as far as you can reconstructing the position and if you can’t go all the way back to the checking move, at least make sure you stop when it is the move of the player in check. If both players are in check for more than the last move, adjudicate a draw.
Alex Relyea
If you can return to the last legal position then do so. If you can return to a legal position both players agree on even though they aren’t sure if it is the last legal one then do so. Otherwise if the player in check is on move then consider the current position as the one where check has just been delivered with the player needing to get out of check, and if the player giving check is on move then undo the checked players last move and have the player get out of check.
I’ve happened to see games where both players thought a check had been there for some time but where it had actually just happened (such as when a player moved an absolutely pinned piece).
With both players in check preferably try to go back to the point of the illegal move for the first check if you can, and secondarily to a point where only one player is in check and is on the move.
Thanks for the help. Without keeping score, that would be hard but we are starting this week with notating moves so it should get easier. I am trying to make sure they learn to play the correct way from the start so there won’t be any bad habits to break later, at least not too many. 
You might be surprised at how much new players can remember. It’s worth a try.
Please see note about both players in check.
Alex Relyea
On the plus side, use the question as a motivation to get them to keep score. I don’t think you mentioned the age group, but irrespective, set a clear and helpful expectation that they must keep score. It’s necessary to play games (well) – see the question they asked, and its also necessary to be able to review their games for improvement.
I’ve OFTEN run into the issue that parents/coaches will say that it’s too hard, so they let kids not do it. I’ve NEVER told them its hard. I explain that sometimes it takes time for people to learn it, and some will learn some things fast than others, but its always necessary. I recall when I first returned to coaching in 1994 and started bringing K-3 teams to tournaments, I had parents and coaches asking me “How did you get them to do notation?” My answer - “We never told them it was hard. We got one or two to do it, and then we noted ‘Hey, your friends can do it. We’ll help you get it down.’ We made it a requirement to play in the weekly club tournament game. Kindergarteners who were still learning to write had a temporary reprieve - but they still had to make an X on the scoresheet for every move.”
With a “labeled” board, nearly anyone can learn to do this fairly quickly.
Don’t worry about it. It will make for funny stories later as the players learn more about the game.
See if the players can reconstruct the game from the very beginning. They may surprise you and be able to do that from memory. Then they see where they went wrong and made illegal moves or left one or both Kings in check.
Don’t expect accuracy in writing notation at first. Player will often forget to write down moves. They will write moves in the wrong column. The wrong file will noted or the wrong rank number used. A lot of moves will be indecipherable. This makes reconstructing games even harder sometimes. If you have a player who has learned descriptive notation, he may mix in both forms of notation on his score sheet. In my experience, it takes players about 4 or 5 tournaments to get the hang of doing notation accurately enough to be usable. Girls are usually better at it than boys. It takes time to learn to juggle the act of thinking, pressing the clock button, and writing down the moves for it to become automatic. Even experienced players have a hard time with this.
When parents express concern about the notation requirement we assure them that accuracy is not part of the rule requirements. That eases their mind a lot. The sooner they try it the sooner they master it.
Complete and reasonably accurate, I believe, is the rule.
I still do these things at the age of 61, after over 30 years of playing tournament chess. So I will vouch for your final comment.