I’m a teacher too and I never give it a thought about how much the child’s family sponsored them for. It is always nice though for the child to have a few names on the sponsorship sheet. It doesn’t matter if the donation is just 50p. Across our (small) school these small amounts add up. On Children in Need days children brought in a few coppers, some £1, unless there was a cake stall 😂.
The most important aspect of these events in school is to instil in children a consideration of the needs of others, and to have a sense of local and global responsibility and their role in that. So the amount of money is not the most important aspect.
There should be no sense of pressure about giving more just because others do. The school will have a number of these events and so your donations will mount up over time.
An extra thought - giving your child £1 to donate rather than say £20 has greater impact on teaching him/her the value of money too, especially at the younger age.