Like ClickyKeys said, multiplication by a scale factor is the key. So... without knowing the numbers offhand nor your present size... let's say a 3-year old is X inches tall, for example, and you are presently Y inches tall. Then you'd want to multiply everything by Y/X to scale it up.
Well, I don't know if there is any sort of "formula" for that specifically.
If you wanted to scale something up or down, you just have to make sure you scale everything exactly the same.
For example, say you want to scale something down to where it's only 20% the original size that is 1/5 the scale. If something was 60 inches in height, just divide by 5. If the width is 30 inches, also divide that by 5.
If you want to scale up instead, you just multiply. So if you want a 10" x 3" object to be scaled up by 300%, That just would be 10 times 3 and 3 times 3. So 30" x 9".
You need to use a ration and labels such as feet = inches
feet = inches
decide what you want to compare and have two labels. Keep you labels in their proper locations, either numerator and denominator.
In teaching junior high math, we used to compute how tall a tree was. If you knew you were six feet tall, and on a given day at 10:00 a.m., you as a shadow of three feet, you could measure the shadow of the tree which let's say is 18 ft. Then you'd set up your proportion as this:
6 ft = x
3 ft 18 ft.
Sorry I couldn't make better looking fractions.
Then you cross multiply and x = the height of the tree.
You're talking about three dimensional objects, so it gets a bit more complicated - doubling height and width will actually square volume. Having said that, for your purposes finding an appropriate ratio based on height and applying that as a modifier across the board will probably work. Based on average heights of 2 year old and adult women, looks like the magic number is a bit less than 1.8.
If you are scaling objects such as blocks (gee, I like that idea, make blocks proportional to my hand compared to a baby's hand) the simple ratio works. If you are doing clothing, you may have to use different ratios for height and width.