There are a few 5 seater taxis that operate from the airport. Perhaps someone on here may have the phone number of the Grenadilla or San Miguel taxi offices & perhaps you can book a 5 seater in advance.
The transfer company prices are a real
rip off. A taxi should cost you approx. 25 euros one way, a transfer company is charging you nearly 40 euros for up to 4 people, that is rediculous.
You may even find a taxi that may take the 4 + the baby, not sure though.
Don't get ripped off & good luck
Jaype