I am somewhat mixed on my kids, not on my teens yet. I aged up my toddlers pretty quickly, because one playthrough, I kind of just wanted to skip that stage so they became kids faster. I put them from long lives to short lives, so it technically wasn't cheating.
Before that, I had used the high chair for their food and drink intake, and gave them all the same thing. There were four at the time, three girls, and one boy. When I aged them up to kids, two of the girls and the one boy were skinny, while one of the girls ended up chubbier. I did the same thing to another four later on, and they all ended up skinny, except for one that ended up average.
I am kind of now letting my toddlers do their thing to get the benefits I need for them later in life, so I have them back on long lives, and letting them get all their goals met, before aging them up. So, I will have to see what they look like later on as kids and teens.