It covers other topics as well as general history but Aquila is lovely, engaging and well illustrated but is geared more towards the 8+ range. She may get some enjoyment from flicking through and interacting with elements but some of the reading may be a bit too challenging at 6 for a reader that lacks confidence.
If she's not yet a confident reader and this is a current interest to be honest I would just seek out some reading level appropriate books to encourage that and see how she gets on rather than a subscription - eg:
Kings and Queens by Marcia Williams
See Inside History if Britain (usborne flap books)
Queen Elizabeth II and the Kings and Queens of Great Britain by Rachel Saunders
Six Queens: An illustrated guide to the iconic queens of Henry VIII by Lydia Monks
DK do an age appropriate series with individual books focusing on particular monarchs ( Henry VIII, Elizabeth I, King Charles and Queen Elizabeth) if she has a particular interest in specific monarchs.
Horrible Histories might appeal (I find some children love it and others not). There is a Top 50 Kings and Queens book in the series but I would place that again in the 8+ category for reading, and it will be mote text heavy than the other options.