I'm not sure by what you mean by "with X-sync" because all electronic flashes are "X-sync" (as opposed to M-sync), i.e. they fire immediately rather than after a short delay. If you mean that it's the only one you own with a PC connector (the little 5mm coaxial thing like a tiny RCA socket), you can get cheap little hotshoe adapters with the appropriate port or (better yet) just get a cheap chinese radio remote which will include x-sync ports and a hotshoe on the receivers - will work with any hotshoe flash that doesn't have high voltage on its trigger pin. If you mean that you don't know how to determine exposure, you can test that with a DSLR if you have one - shoot a frame, adjust the flash power or aperture, repeat until exposure is good then expose your film with the chosen settings.
Try using some grazing light on the baby's face to produce specular highlights. And try to ensure it's not wearing bright white clothing.