First - and you may see conflicting suggestions on this subject - you should avoid simple USB plug adapters when using USB-C cables. The USB-C Certification Standards do not certify plug-adapters - principally as they can introduce incorrect connection combinations that can damage the USB-C port of the host devices. Certified USB-C cables incorporate active electronic components that protect the cable and connected devices from damage - these components being missing from plug adapters. USB-C cables also meet much more rigorous technical standards.
All components in a USB-C connection chain - Power Adapter, cable and Host device - are actively involved in determining the charging voltage and current supplied to the host device for charging. Older USB standards use the basic (and much less efficient) USB 5V charging standard. USB PD Power Adapters will automatically default to using the older charging standard when older devices are connected for charging - and as such can be safely used with your older iPad and iPhone when used with an appropriate USB-C to Lightning cable.
While “plug adapters” are widely and cheaply available from multiple sources, ask yourself this. Is it worth risking potential serious/expensive damage to your electronic devices so as to save a little money in avoidance of purchasing the correct cable? When compared the cost of your not-inexpensive iPad, the cost of a good cable, designed for the job, pales into relative insignificance.
Also be aware that Certified USB-C cables are generally limited to 2m (~6’). If you need more length, consider extending the AC-mains side of the equation in preference to using longer and more fragile USB cables.
Whilst perhaps not immediately relevant, should you ever consider purchase of a replacement Power Adapter, ensure that you choose a USB-C Power Adapter that supports USB PD (Power Delivery). Most Apple devices will optimally fast-charge when using USB-PD; other charging standards, such as QC (Quick Charge) and other proprietary charging standards are not supported by Apple. Be aware that some third-party manufacturers produce Power Adapters that claim to support multiple charging standards; of those, some don’t work well with Apple devices, if at all.
