How to adjust a knitting pattern

How can I adjust a child’s chest size from a 30" to a 34"? any would be appreciated. Thanking you in advance Linda

Hi Linda and welcome!
The best thing to do is to knit a swatch with your yarn and needles and figure out how many sts you get per inch. Knit about a 5x5 or 6x6 inch swatch so that you can measure over the middle 4 inches. Then you can see what the knit fabric looks like and see if you need to change needle size. It’ll also help you decide how many sts you need to add in order to get the chest measurement you want. You’ll also need to adjust the length and the proportions of the sleeve if you change the chest measurement.