Answered by Mufti Mohammed Tosir Miah
Doubts regarding cleanliness
There is a principle in Islamic jurisprudence which is:
“Certainty is not lifted by a doubt.” (Al Ashbah Wan Nadhair p.60)
Therefore, with regards to cleanliness, you should presume everything to pure and clean and if you have the slightest doubt regarding something being impure, it is obligatory for you to consider it to be pure.
With regards to your situation, like it is mentioned in the answer it is obligatory for you to consider the items to be pure except where you have seen with your own eyes the items becoming impure. Only then you will consider the items to be impure. Similarly, it is not necessary for you continuously or even constantly keep an eye on the pure or impure hand of a child when it touches you and then doubting whether I have to wash this part or not.
Only Allah Knows Best
Mohammed Tosir Miah
Darul Ifta Birmingham