How did sanatkumars reach Lord Vishnu's abode? Sanatkumars were also incarnation of Vishnu, but they were only children, how did they reach there in such a small age? This is a difficult task. What technology they used?
You are asking how Sanat-Kumaras reached the abode of Lord vishu – what technology they used?
The answer is given in Shreemad Bhagvat Purana Skandha 3 Chapter 15
These verses tell us how Sanat-Kumaras used to travel in celestial regions
ब्राह्मो उवाच मानसा मे सुता युष्मत्पुर्वजा: सनकादय : |
चेरुर्विहायसा लोकानलोकेषु विगतस्पृहा: ||१२||
mānasā me sutā yuṣmat-
cerur vihāyasā lokāl
Lord Brahmā said: My four sons Sanaka, Sanātana, Sanandana and Sanat-kumāra, who were born from my mind, are your predecessors. Sometimes they travel throughout the material and spiritual skies without any definite desire.SB 3.15.12
This verse tells us that they used to travel throughout the material and spiritual skies. The Gita Press version tells us that they were traveling through Skyway (Akash-Marga).
Now look at this verse
तद्विव्श्रगूर्वधिकृतं भुवनैकवन्धं |
दिव्यं विचित्रविबुधाग्र्यविमानशोचि: |
आपु: परां मुदमपूर्वमुपेत्य योग |
मायाबलेन मुनयस्तदथो विकुन्ठम् ||२६||
tad viśva-gurv-adhikṛtaṁ bhuvanaika-vandyaṁ
āpuḥ parāṁ mudam apūrvam upetya yoga-
māyā-balena munayas tad atho vikuṇṭham
Thus the great sages Sanaka, Sanātana, Sanandana and Sanat-kumāra, upon reaching the above-mentioned Vaikuṇṭha in the spiritual world by dint of their mystic yoga performance, perceived unprecedented happiness. They found that the spiritual sky was illuminated by highly decorated airplanes piloted by the best devotees of Vaikuṇṭha and was predominated by the Supreme Personality of Godhead.SB 3.15.26
This verse says that they have reached the Vaikuntha by dint of their mystic yoga performance i.e. (YogMayaBala) (Yogic Powers). So they haven’t used any technology but their own yoga power to reach over there.
You can read more about the story in this chapter of Bhagavatam.