Have you tried this query:
project = ABC and issuetype = Epic or parent in (ABC-1, ABC-2, ABC-3)
^ This gives you all the Epics within a Next-Gen project plus all the child issues of a set of Next-Gen Epics which you define.
Parent is used here rather than "Epic Link" as it's Next-Gen.
The ABC-* references each Epic singularly, providing the children of each. This is useful if you only want to locate a selection of Epic's children via issue search.
If you just want all Epics and their children which are displayed on the roadmap then you could use:
project = ABC and issuetype in (Epic, standardIssueTypes())
^ Or you can customise the issue types if you don't want all standard types - for example to just receive Epics and Stories:
project = ABC and issuetype in (Epic, Story)