Thank you for your query.
It is very very rare to have thrombosis
in such a small child without her complaining of anything.Just check out whether she has been bitten by a bug or not.What are her complains? Is her pulses all right behind the ankle? Check her carefully first.If she is asymptomatic you can observe her carefully.If she develops any symptoms it is better to attend ER.Hope this answers your query.Wishing your child a speedy recovery.
Have a nice day.