We have been working with the team at Jelurida (developers of the NXT and Ardor platforms) to implement support for child chains IGNIS and SWIFT.
The recommendation from the Jelurida team for sending child chain transactions is to use the AccountPropertyBundler feature. This bundler will bundle transactions from any account on which the bundler account has set the "bundler" propery.
The AccountPropertyBundler feature will be in the next version 2.0.13 of Ardor. The Jelurida team anticipates that they will have the new version out in the next few days. Bittrex will then perform some further and testing and integration work.
Thank you for your patience.