Can I use this memory for a computer build?

I’m building a computer for my 6 year old for Christmas this year, I will be using a Pentium D 805 processor from my older computer and everything else will be brand new. She does not need anything fancy I’m just building her a basic machine to play her games and some learning programs on. I was wondering about memory though..I have some RAM from the old computer that is 4GB 533mhz PC4200…the motherboard I bought her has a memory standard of 800mhz. Would I be able to salvage the 533mhz and use it in her computer, can I go down from 800mhz memory just not passed it? I’m just trying to salvage anything I can from the older computer.

again this is not about power or how fast anything is, just a basic build