All Purchases made on the AppStore can be downloaded again for free to any other device that you own. To do this you must be using the same Apple ID for App Store purchases on all your devices.
If the device you wish to sync with uses a different Apple ID to the one you made the original purchase, you will be required to pay for the App again.
All In-App purchases, including sync support, made within Baby Feed Timer will be honoured across all the devices you own without having to pay again, so long as the Google Play account on those devices is the same as the one that made the original purchase.
If you wish to sync with a device that uses a different Google Play account then you will be required to purchase the sync feature again.