I planning to buy brand new iPad and tablet from the UK as I live here to take to Thailand for my cousin and my grandchild.
Do I need to pay for custom tax?
Technically, yes. From here: "Some products can be imported free of duty, e.g. laptops and other electronic products." and "Goods imported into Thailand are subject to VAT at a rate of 7% calculated over the CIF value plus any applicable duty." with CIF being "Cost, Insurance and Freight". So, you do not have to pay duty, but you would have to pay sales tax.
That said, you're not likely to be stopped for this. But, to minimise the risk, make sure the boxes are opened and, better yet, get rid of the boxes altogether.
However, as a foreigner, it's possible to get the sales tax back on electronics bought in Thailand. It might then be cheaper to buy the products over there.