What anaethesia to use for biopsy of growing black spot under tongue of a 5 y/o?

Talk with an ENT. Biopsies at this age can be very hard to do. I would recommend having a little sedation in the operating room, so that the experience is not traumatic for the child and it is doe safely.
Chloral hydrate? Chloral hydrate is still commonly used as a sedative for children before procedure. Since only small amount of tissue is needed, local injection of Lidocaine may be used after child is sedated to get the sample. Anyhow, this is my guess. I suggest talking to/asking the doc/surgeon who is doing the procedure to find out what will be used/done etc..Good luck.