Children are developing their understanding of combining two groups, from the moment they understand what 'add' means.
We have explored this concept in many different ways through play -
hoop games (how many are in the first hoop? how many are in the second hoop? so how many are there altogether? in total? Repeating the same language is really important too.
I use lots of vocabulary when teaching addition. Add, together, amount, total, amount, altogether, groups, part, part, whole.
Using hands/ fingers is a really effective way of teaching adding under 10. " I have 3 fingers on this hand... and I have 4 on this hand... 3 is one part... and 4 is the other part... when I add them together I count all of the parts and this makes the whole which is 7.
3 + 4 = 7