A child at 30 lbs is between the ages of 2 years old to 3 years old.
2 year old girls weigh between 19.5 lbs to 32.5 lbs.
And 2 year old boys weigh between 21 lbs to 33.5 lbs.
3 year olds weigh between 26 lbs to 38 lbs.
4 year olds often weigh between 30 lbs to 44 lbs.
So the weight of the child not only depends on age but their build and individual growth rates which can vary from child to child.
But on average a 20 lb child is around 2 years old to 2 and 1/2 years old to 3 years old.
Per the CDC growth charts, a 2 year old is often between 20 lbs to 35 lbs but you have to also take into account the child's height as well when determining the child's weight.
Weight for stature or weight for height charts can also help with the weight for height.
My 2 year old weighs 27 lbs which is normal for him and he is just a skinny but healthy child although my nephew was 30 lbs at 2 years old so it can vary from child to child.
Some kids even when they're eating enough are lower in weight than others and some kids are more skinny than others are as well.
As long as your kid is eating well and healthy it's okay for them to be slightly underweight than other kids the same age or slightly over the weight of the same age of kids.