Try as I may, I cannot think of a suitable activity to go with this concept. This one really boils down to us parents role-modelling the correct way to do good and then not spoil the good that we have done.
Spoil it how? Allah says:
“Kind speech and forgiveness is better than charity followed by injury” (2: 263) and “Do not make your charity worthless by reproach and injury” (2:264)
If our children see us helping someone when they have asked for a favour, but then complaining about it to others…
Or slipping it into conversation with them about how it doesn’t matter but it took ages to do something for them…
Or helping someone but then indignantly telling our spouses how they didn’t even thank us…
Or sighing and only reluctantly helping someone…
All these things will show them that helping others and doing good deeds (forms of charity) are a burden, and not the positive opportunity that it is.
When an opportunity for them to do something good comes up and they don’t seem so keen (even for something as small as getting something from another room for their sibling), remind them that this is a gift from Allah and a chance to score some happy points! How unlucky is the person who does not even get chances to do good!
If they do it but then complain, or don’t do it happily (e.g. throw what they’ve gotten to their sibling in a huff), remind them of the above ayahs and tell them not to spoil what they have done…
Perhaps the analogy of an apple might work here. A lovely ripe apple is like a lovely good deed that you have done. When we do it reluctantly, or complain about it, it is as if we are throwing the apple down hard on the floor and bruising it.
What is better? A fresh smooth apple or a bruised rotten apple? Food for thought…