Something needs clarification.
20 grams? Kaa I am assuming is a BP and uhhhh I've not seen a snake that small come out of an egg.
If It was a typo and 200 grams which sounds more reasonable is what we are dealing with going 10-15% of body weight gives 20-30 gram prey.
2 rat pinkies are probably underfeeding for a snake of that size although, suppliers can and do differ in what they class as each size.
If this is a 200 gram snake I would definitely increase prey size. This also helps them stretch jaw ligaments as they grow.