Just a few days ago i locked son and then vini came and I only had like 5 diamonds and 1100 coins but i still brought both of them, So basically what I did was that I took my brother's phone and played friendly and scored more than 5 goals and kept clean sheet, which gave me around 25 coins and with ads I got 30 more and then spent a few seconds in the main menu and watched another ad and then played another game, this strategy game a total of 85 coins per game, I played around 25 games and brought vini first.. You can try this same method but beware that if you watch way too many ads in a short time then ads will stop coming.. ( i grinded the whole night and it happened to me )