A household, not a single applicant
People picture this as one document and one translator. In practice a move to Taiwan tends to involve a spouse, sometimes children, occasionally an elderly parent, and every one of them arrives with their own small stack of paperwork. Treating it as a household job from the beginning avoids the pattern where one person is ready and the rest of the family is still hunting.
The language Taiwan requires
Chinese is what is needed for documents used in Taiwan, and each person's record goes across in full. Names, dates and relationships have to line up across the different papers, which is far easier when the household is written in a single run by one certified Chinese translator than when the documents are produced weeks apart by different hands.
Certification Taiwan accepts
Each translation is certified with a company stamp, which authorities accept, so every member of the group is covered by the same standard of certification. There is no need to arrange something different for a child than for a working adult. Where the receiving body additionally asks for notarisation, that can be arranged on top for whichever documents need it.
Per-page cost and turnaround
Budget by counting every page the household holds, at £25 each, which gives a total for the family rather than a series of surprises. The wait is 3–5 business days from clear images, or 48 hours on the express route. Sending the whole group at once means one wait for everybody instead of a queue of separate ones.