Can childeren that just started there period wear a tampon?

Yes. However, i would not recomend that initially. I think starting out with pads first and then slowly introducing them over the next year or two. Also depends on age and maturity.
Physically, yes. Physically, a girl who has a menstrual period can use a pad or tampon. However, some girls are quite young (ages 10-12 years) when periods start, so they may not be comfortable inserting or removing a tampon (parental guidance and help is needed at first). Tampons can have complications if not changed often enough. Young girls may not be mentally mature enough to remember to change tampons often.
Yes-if child wants. Tampon use if fine in girls who've just started having periods-if the girl wants to. It's important to teach your daughter about toxic shock syndrome, the importance of changing tampons regularly, not just when they are saturated. Change tampons at least every 4-6 hours.