Find your child's bounty on Subsquare under our bounty #43. Open it and in the right bottom part click on Claim.
Go to polkadot.js - Developers - Extrinsic and fill in the details that are on this picture:
A number of your child bounty can be found, for example, via Subsquare here.
Keep in mind that bounty claims can be initiated by anyone. In some cases, curators might do it for you.