I think that is part of it, I’m not sure it’s true though, I think most people only get one kit, if at all, possibly those with kids have more “pester power” when they are released like this.
I think the other thing is cost, priority and demand, look at clubs like Real Madrid, all their 3 kits come out before the season starts, they get priority at the factories that make Adidas kits. Adidas prioritise getting these out before the season starts, because they know they will sell millions of them and make a fortune and pay Real Madrid a fortune.
Realistically if all 3 of our kits came out on the same day I don’t think it would make people buy more or less shirts, but it would cost umbro more to produce them, because that period between May and July will have the highest demand at the factories, from football teams all over Europe for their 3 kits.