Regarding the bus. The bus only runs during the day from the airport. To get from Santa Eulalia from the airport, you would take the bus to Ibiza Town and then change at the main interchange to the bus to Santa Eulalia. Full timetables here - http://www.ibizabus.com

Unfortunetly I don't think you would find it very wheelchair friendly (if the child is fully wheelchair bound) as they are coaches oppose to buses. However what that does mean is that there is usually storage on the buses, so providing your child can get on the bus without too much stress then storing any mobility equipment should not be a problem.

I think a taxi will cost you in the 30 Euro region one way. Registered taxis are metered (white cars with a green light on top) so you should not worry about been fiddled. In Ibiza it is perfectly legal for taxis to stop and pick you up if you flag one down. Be careful of mini-cab drivers, they are often unlicienced and a law on to themselves.

There are a few transfer companies on the internet, such as http://www.transferstore.com (whom I have never used!) (or try this Google search) who will pre-book a transfer for you, both too and from your hotel if you wish. However they do tend to be more expensive than just jumping in a cab, however you would be able to phone them and let them know your requirements prior to booking, which would give you peace of mind.

I hope you have an enjoyable holiday.