Hi Miriam!
I agree with Noha - using the flash cards twice a day is fine, no problem about that.
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The lovely thing about teaching little children (and all children - both young and old) is that learning is always subjective; there's no exact science to it because all kids react in different ways during different times. Some children will want to see flash cards only once a day, some don't like it during some days, and some are insatiable, wanting to have a look at different cards very frequently.
The most important thing to remember in all this is that we should take our cue from the children - if they are distracted or beginning to get bored, then we should put the cards down and stop.
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The whole point of teaching is to make learning a fun and enjoyable task for them, and if they aren't having any fun, then we should take a step back and rethink our process of teaching and find some ways to spice it up so that they can look forward to it.
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We have a lot of working parents in this forum too, and I think that it's great that we are all looking for a good sense of balance between work and time with kids. Teaching can be a fantastic way of bonding with your child, especially if both the parent and baby are having good fun in the process.