What is this small tree?
This is very likely some species of Viburnum.

Viburnum is a genus of about 150–175 species of shrubs in the Adoxaceae family that are primarilly native throughout the temperate Northern Hemisphere. 

All species of viburnum (as far as I know) have opposite leaves, as does the specimen in the picture.

Many species of Viburnums bloom in white in the last part of April through early May, and typically you have to pay attention to flower shape and often, more importantly, the leaf shape to differentiate species.  

Without location information, an exact species ID is not guaranteed, but your specimen looks very similar to: 

Doublefile Viburnum (Viburnum plicatum)



 Source: Dave's Garden 



 Source: Missouri Botanical Garden 

This species produces two rows of white flower clusters, with both small and large flowers, along the stem. 

According to Wikipedia: Viburnum plicatum is a popular ornamental plant, both in its native area and in various temperate regions.

Based on the "double" flower types of your specimen, it's likely V. plicatum f. tomentosum: 


  Cultivars with wild-type flowerhead structure are sometimes described as a separate botanical form V. plicatum f. tomentosum. They include 'Cascade', 'Lanarth' and 'Rowallane'.2[3] Two cultivars in this group, 'Mariesii'[5] and 'Pink Beauty',[6] have gained the Royal Horticultural Society's Award of Garden Merit.


Habit: A deciduous, multi-stemmed medium-sized shrub that typically reaches 2.5 3 m tall. [Source].

Leaf: ovate, 5-10 cm long, up to 5 cm wide; pointed apex and rounded leaf base; serrated leaf margin; 8 to 12 pairs of veins; pubescent underside; dark green leaf color. [Source].

Flowers: individual flowers form a large (up to 10 cm diameter) flat-topped cyme composed of showy infertile flowers surrounding unshowy fertile flowers. Blooms in May. [Source].