There would be no addition to their allowance for them to play poker, and by giving them an allowance, the incentive would be for them to buy whatever toy/game/candy they wanted and save a portion to invest in the long term.
If they wanted to play poker they would have to understand that the money lost there would be a toy/game/candy that they could have bought and won't be able to. I wouldn't stop them from playing for the following: if they were good players, they would start making money at an early age. If they were bad, they would have to learn from an early age to deal with their frustrations, and difficult moments make strong men, childhood is the ideal time to teach someone how to deal with difficult moments.
Adults who do not know how to deal with difficult moments are like that because they were not subjected to such an experience as children, in which they could have already learned something.
NOTE: If you are an intelligent child who learns easily, if you are the one who is in recovery, you don't understand mathematics, without a chance, at all.