Can a person who has performed Prapatti attend salangai poojai (ceremonial worship of anklets) as it is performed to Lord Nataraja? Will this participation lead to associating with a demi-god?

Salangai pooja is done to honor the relevant Lord of dance, it is not harmful to avoid but you can go for the sake of your child and need not participate in worshipping, after all, there is no need for a Prappanna to worship demi-gods.