Hi Hussain,
1. Don't think yet the device has been approved for usage on patients below 18 yo. Some others might know this? So when ordering it, you will have to quote it for your own usage. But technically, I think it was designed from day1 with also paediatric care mind. The sensor diameter is around a 1 Euro/UKP coin, so should also fit on a smaller upper arm without much trouble
2. If otherwise that Abbott is not out of stock/in backorder situation, then delivery is typically max two weeks from when first signing up and ordering completed. You should therefore have ample time to order and try it out during your 4 months stay.