It's a two-way street, gambling when they're awake and lying to them about what you're doing, which is not quite a lie though, he's actually playing a game. Or gamble when they're asleep and sacrifice your sleeping time for the sake of gambling. Many would go with the first option, as it's not a difficult task to tell the kid what we are doing, on the screen. Gambling is a game; the only difference is the amount of money we wager to play the game. People gamble in PlayStation games, it's still gambling and makes no changes, from the definition of what we have online. So, distracting the thoughts of the child is a smart move, but doing it moderately helps in stabilizing the curiosity of the child. Their brain is fast and remembers to some extent what we do and feel after playing the game.
A child may ask, dad why do you look so serious when playing the game, or your face looks sad whenever the game is over, why? These question as we may have it, came as a result of observation. If they watch you gamble, they'll master the facial reactions, and it wouldn't match with how they feel when playing a video game. The questions won't end, and we must be ready to keep on answering more query every day. I'd advise for the gambler to take it gradually, and gamble when the kids are not around us, as Dewi Aries said. The hardest aspect of it, is what we'd do, when the child wants to sit with us while playing the game. Kids can be worrisome, but we have nothing to do about it, then oblige to their numerous requests. You see it'll be wrong sharing same screen displaying a casino site, with a child. The child may begin to memorize what he's seeing on screen, and it's not conducive in my own view.
I think it's fine if you lie for the sake of something that leads to goodness because after all that's the best thing to do if indeed you are one of the fathers involved in gambling, and don't let you be careless in taking precautions so that your children's curiosity does not develop, plus it's clear as you said that a child usually has a sharp memory and also has a high enough sense of curiosity that this can make them find out what exactly you are doing and it is possible that they are involved in gambling without you realizing it. After all, as you said, there are still quite a lot of prevention options that we can do, one of which is in terms of dividing time, you can gamble at your children's bedtime or gamble in other places that are essentially far from your child's environment.
Well of course such a situation is also very likely to make a child become very curious about what exactly his father is doing, they will be very curious and then ask you some questions and if you always make the same mistakes in the sense of not paying too much attention to the importance of the prevention aspect then obviously the suspicion of a child will be more developed, so always keep this activity away from the environment and the view of a child if you really want the best for your child.
Those lies could help, but not forever. Sometimes humans forget what they said previously about a certain question. When that same question is repeated other times, weeks later, the recipient of the question may not give an answer that matches his previous answer, if he lied. The truth sticks to the brain, and we don't need to bother about fabricating something to say. They'll be nothing to lie about computerized objects moving in the screen. The child sees the avatars, and characters in the game. S/he may be wondering why daddy is seriously playing such games, that looks like it's meant for kids. Looking at it they know it's a game, why a grown adult is playing such a game.
Actually, the child may not be satisfied with the answer; truth or lies. So, they'll have to asks again some other times. When it doesn't correspond, the child may remind us of our previous answer, the lie didn't match. Creating more confusions. I think they'll should be more explanations to do, when in such a condition, a child being inquisitive of our gambling activity. Drawing an image, in the child's mind, of the quality and importance of the game, without mentioning gambling, that made you to engage in the game every day. Explanations makes confusing things clearer and brighter for a child. Let them see a reason. It will reduce the rate the child repeats his or her questions. While at it try diverging the discussion to other life situations and bringing it back to the main topic. With a good interpersonal skill, the gambler doesn't need to hide and play gambling. Unless the child distracts us from being attentive in the game, changing location will be fine.