It is fine to use kegel balls with an IUD. When you inset your kegel balls they should sit below the cervix inside the vagina, above your pelvic floor muscles and they definitely should not come in contact with the IUD itself.
If you plan on using larger sex toys, take into account the threads on your IUD. The shorter they are, the less of an issue they may be when you’re using toys. You can tell your gynaecologist about your sex toy usage and ask them to trim the threads back for you to a length that won’t get in the way.
With any sex toy you frequently use, always watch out for anything that might promote infection — sex toys and body parts entering your vagina need to be clean, you can use condoms on your sex toys to take extra precaution.
In all cases take caution and consult with a qualified medical practitioner beforehand.