We don't deliberately let a child win (of any age). These are the strategies we use:
1) lots of games we play are games of chance, like Trouble or Snakes & Ladders.
2) we always play a game that suits the skills of the youngest player (so for eg no chess with my 4yo, since she can't play yet).
3) the first few times we play a game we play it 'open handed' and don't keep score, until everyone knows how to play.
4) we help players who need it, by offering advice or strategy.
5) we sometimes play modified rules according to the age of the children. Eg No 'going back to start' in Trouble, not playing the bonus points in Scrabble.
We think it is important for them to have the achievement of really winning, even if that takes years to achieve. Particularly important for strategy games. I refused to even play Scrabble with DS until he was 7yo, because I knew it would be too humiliating for him

. Now he can score plenty of points but still not beat me, but he can see the improvement, and is sure that every time will be the winning one

.
The first time I played chess with DS (I'm not a good player at all, he always plays with DH) was when he was 7yo. He almost beat me in 3 moves

. It was only that he was grinning his head off and DH was giving me warning looks that I realised what he was setting up. The game ended in a stalemate and he has beaten me since. Boy does he feel proud!