I think the most important thing is that William keeps talking about his mum, showing as little tears as possible.
I lost my grandparents by the time I was 7, and I never met my grandad. But my mum has always been so open about sharing memories and stories, and in a small way they've become like my memories too. But whenever she cried about her parents while I was a child I wouldn't talk about them unless she brought them up as I was fearful of upsetting her. So through her being able to share funny stories or remarking similar traits I have to them, we've got to a really lovely stage where we can talk about them without getting too visibly upset, whilst also having times where the grief just hits.
This looks like quite a fun activity, with there being dialogue about Williams mum and how he feels. The kids (and Kate) are learning about their grandmother from someone who knew her the best, and are developing an emotional connection through a creative, thematic exercise like card making. It also helps to normalise these feelings of love and loss (if they can ever be normalised, I mean that its allowing an openness and breaking the taboo of talking about a deceased person which was once a popular approach).
And believe me, you can definitely grieve for someone you've never met. Also grieve for the fact you never got the chance to meet them.
I also like the dress Carole is wearing, I know it would be a fashion trend at the time, but it reminds me of the dress William's mother wore when she left hospital, with the beckoned and shape. It looks like Carole is pregnant with Pippa too perhaps? And definitely a lot of Louis in young Kate, also Charlotte too, who I always think of as William 2.0