Children will be learning at school how to round up decimals to the nearest whole number. A fun way to practise this at home is to use regular dominos and establish that each piece is a decimal number (so a domino one with 3 dots and 4 dots is 3.4). Encourage them to play with the dominos, but with the rule that they can only, for example, join 2 dominos that are within 1.5 of each other. This will really get them thinking about what each decimal point represents. A number line can also be useful when rounding decimals.
Practice with worksheets. Once you have mastered the flash cards, try your hand at some worksheets. Start by practicing with one number set at a time. When you have mastered all of them, try worksheets that mix up all of the number sets and see how you do. If you know your tests will be timed in school, try practicing with a timer.