Watch a first grade student playing a game that involves rolling dice. Probably you can look at the outcome of rolling a pair of dice and immediately say the total. The first grader may need to carefully count one die and then keep going with the second. The transition to the level of expertise you have comes from practice. People who advocate use of games in math education want the practice to be fun and to include additional learning activities.
For example, suppose only one die is rolled, and it comes out 4. Is it likely that, when a second die is rolled and added to the first, the total will be 11 or higher? (This is a tricky question—you want to challenge the child.) That is certainly a challenging question to most first graders. Answering it takes some understanding of the number line and some practice at doing mental arithmetic.