@vyuschuk, there are several great things about VEX EDR system.
First, there is a lot of matched mechanical components that are very easy to build / prototype with, if you need a platform that is stronger than Lego Technic or VEX IQ.
Second, if your kids are into robotics and are competitive they may fall in love with VRC and there is a great community that formed around it.
Also, VEX EDR electronics is good for students to get started with out of the box, but it was not designed for easy tinkering or to be interfacing with non-VEX components. Legacy Cortex system is slightly better in that regard, but V5 is nowhere near to the point where it is as open and as well documented as something like Arduino or Raspberry Pi.
The best document, I’ve seen, about interfacing with non-V5 components is the one linked by @majorjoel2
However, it is not for the faint of heart and you will need to have a lot of prior electronics knowledge just to understand it and get started.
If you already know that your children like to tinker with metal parts, screwdrivers, gears, and wheels then VEX EDR V5 could be great platform for them. And if they happen to like V5 and/or get into competitive robotics, then expect yourself to be buying more mechanical parts over the next months and years. But, if you are not sure about their future commitment, then VEX IQ or Lego Mindstorms might be a cheaper alternative to collect dust after a few weeks of initial use.
Personally, I like VEX EDR mechanical system for quick prototyping and getting demonstration models built, but V5 brain and other electronics is too expensive to leave it on a one off models like people usually do with Arduino or other cheaper alternatives.
Unlike V5, older Cortex motors and electronics are much easier to interface with other control systems (Raspberry Pi, Arduino, ESP), which may be something for you to consider.
Here is an example of using EDR motors and mechanical components with NodeMCU/ESP8266 microprocessor module controlled by a cell phone over WiFi: Inexpensive control system for teams without access to Cortex / V5 off-season
If the budget is limited you can get your feet wet in VEX by getting an older Cortex system cheap on eBay, and then, if you like it, keep adding more parts and/or V5 control system if you decide to compete in VRC.