The order in which gifts are received doesn't matter - if child X gets toy 1 then toy 2, it's the same as giving child X toy 2 then toy 1 - so we are counting combinations.
The eldest child receives 3 of the 7 gifts, so they have
possible choices of gifts.
The next child receives 2 of the remaining 4 gifts, so they have
choices.
The last child receives the remaining 2 gifts, and there is only
way to select the gifts for them.
By the multiplication using, the total number of ways of distributing 7 gifts among 3 children in the prescribed way is 35 • 6 • 1 = 210.