I’ll third tramantuna tours – but watch their seat posts (v long!). The Barranc de Biniaraix descent is great, but hard work on the arms. An easy and fun family trip is Balitx valley. You can ride along the entire tramantuna range, staying in pensions – never done it, but would love too! Getting an uplift to the top of the american road is worthwhile as it is unrelenting.
If you go the other side of the island, try roxy-bike in cala millor – this side is not as mountainous, but routes are great and v technical partic around son servera. If you are staying in Soller this is a bit of trek though (1.5h, but beaches too and if you’re there for 1m then worth exploring the east as kids will love it!) These guys also rent bikes, some lovely trail riding near Arta.
I agree the best trail guide is http://www.rockandride-mallorca.com their booklet (photocopy special) is well worth it, Tramantunatours sell it – a gps is helpful as trail marking is not great and rights of way variable.
Mallorca is a great mtb destination, not just for roadies, just watch the rain as the rock gets like ice so bring pads!