code: public static SendRequest childPaysForParent(Wallet wallet, Transaction parentTransaction, Coin feeRaise) {
          TransactionOutput outputToSpend = null;
          for (final TransactionOutput output : parentTransaction.getOutputs()) {
              if (output.isMine(wallet) && output.isAvailableForSpending()
                      && output.getValue().isGreaterThan(feeRaise)) {
                  outputToSpend = output;
                  break;
              }
          }
          // TODO spend another confirmed output of own wallet if needed
          checkNotNull(outputToSpend, "Can't find adequately sized output that spends to us");
  
          final Transaction tx = new Transaction(parentTransaction.getParams());
          tx.addInput(outputToSpend);
          tx.addOutput(outputToSpend.getValue().subtract(feeRaise), wallet.freshAddress(KeyPurpose.CHANGE));
          tx.setPurpose(Transaction.Purpose.RAISE_FEE);
          final SendRequest req = forTx(tx);
          req.completed = true;
          return req;
      }
docs: Construct a SendRequest for a CPFP (child-pays-for-parent) transaction. The resulting transaction is already
  completed, so you should directly proceed to signing and broadcasting/committing the transaction. CPFP is
  currently only supported by a few miners, so use with care.