While not a prayer app,
"RemindMe" is a very flexible reminder app for recurrent tasks, such as saying an "Ave" at the top of the hour.
I find that building on humbler, smaller tasks such as with prayers and devotionals tends to work better, at least for the acedia prone such as, sadly, myself.
Also, for those more pressed and stressed, such as mothers, you can always check notifications for that missed in order to better tend to your duties without "sweating it"
I also notice, like a little planting, that it is almost "automatic" to build over time, such as making a good signum, adding little ejaculations, meditations and contemplations without really even 'making' myself do 'more'.
It also helps to set these reminders for recollection, such as saying a quick ave before and after posting here to mitigate error and to enhance.
The recollection really comes in because you find yourself more aware, of "counting the minutes", and so not straying so much into forgetfulness and if you lose custody, you aren't nearly so far gone.
(I know, but if you think I'm bad now...)